Hate to say it, but I don't think you have just one issue to solve with your tank. I went through some of the same when I inherited a salt water tank a few years ago. It had high nitrates, high phosphate, full of aiptasia, and I was a SW newbie. Needless to say, it took me a while to figure things out, and I had to change a lot of things before it got better. Good news, I was successful, and now have a very healthy tank with zero nitrates, and near zero phostates, and growing SPS. Oh, and no aiptasia.
It does come down to basics though. Is your equipment good? What are your husbandry practices? Do you have the right fish and CUC that help keep your tank clean? And chemistry? Test kits? Go slow. Fix the right problems first. Monitor your progress and confirm what you are doing is working.
Lots of good reading on the web, including some great articles by Myka. Do some searches and should be easy to dig up all kinds of stuff on almost anything. Here is one example with an index of topics.
http://www.reefkeeping.com/issues/subject/index.php
Of course, don't believe everything you read, and the forums are full of opinions. But with knowledge and experience, you should be able to figure out what and who you can trust. Plus there are many methods that work, and there are always new ways of doing things. That's part of what makes this hobby fun.
So where do you start? I would first look at your equipment. I believe you have a 180g tank. How much flow? I have a 100g and have 6500+ gph inside the tank. Strong flow helps keep the rocks and sand clean from detritus, where algae likes to grow. You said you have a new skimmer. What is it rated for? While my tank is 100g, I have a skimmer rated for 280g. Do you use filter socks? For that size tank, I would use a 7" inch diameter (16" long). If you have room in your sump, I would use two of those. That helps collect particulate matter in your water before it has a chance to turn into nitrates and phosphates, and helps make your skimmer be more efficient with less cleaning. RODI is a given. Various media reactors are also useful and I will talk about these later. Oh, and I would ditch that canister filter. It can be a nitrate factory.
Good husbandry is also most important. I would go with 20% water changes every 2 weeks until your tank is where you want it. Then no longer than once a month, if all your parameters stay good. Despite what some people here say, I would recommend vacuuming your sand with each water change at the beginning. I did that, and was amazed how much crap came out of my sand. Later you can do it once every few months to prevent build-up. You can also do different sections of the tank each time. A shallow sand bed also makes things easier. And keep picking away at that algae. I used a toothbrush on the rocks, and blew off the detritus with a turkey baster. Thankfully that effort became much easier and less frequent as my tank got cleaned up.
The right fish can help too. I have a yellow tang, kole tang, lawnmower blenny and a foxface that are always picking algae from the rocks and glass. I also have a diamond goby that does a wonderful job of keeping my sand clean. Some snails do work, but since I got some hermits, they were all done in. However, I have these little stomatella snails that have multiplied in my tank, and do a great job of keeping my upper rock surfaces (where the light is really strong) polished clean of algae. And the hermits don't bother them. Stay away from conch and other sand sifting snails. I tried them, and they didn't work for me. What's worse, is they get stuck under rock work and die there, making things worse. And whooh, what a smell when you remove them...
Media reactors are another great help. I would start with a bio pellet reactor to get your nitrates down. Here again the right equipment is very important. You want one with a lot of capacity and flow to keep your pellets tumbling properly. I use a Vertex UF20 and it can handle a full 1000 ml bag. However, it is important to follow directions and start slowly. The use of MB7 is also very important. I would start dosing heavily with MB7 (and you don't have to shut off your skimmer) a couple weeks early. It adds beneficial bacteria that helps prevent cyano, mulm, clumping and other bad things that can happen with bio pellets or carbon dosing (eg vodka). Once your nitrates come down (could take several months), then I would add GFO. That will take your phosphates down to near zero. I use an old converted RODI 3 cannister system (similar to the BRS GFO reactors) and run both carbon and GFO. That has worked well for me.
Oh, I should mention, I did initially try a refugium full of chaeto. It didn't make a dent in the nitrates or phosphates. I guess my tank at that time was just too far gone for it to make a difference.
Anyway, it can be done. Go slow. Don't expect immediate results. And don't give up. And nothing more gratifying afterwards to know that you won the battle. Good luck.