Crazy. I just went through the same thing. I was dosing vinegar and began running uv... Just because I had a uv light... Cyano showed up and was getting quite bad. Took away uv... cyano has been steadily decreasing and breaking down. Thick gunk in my skimmer again too.

My new opinion on UV. UV = bad. lol

Ran UV for about 6 months as I had a suspicion there was a communicable zoa disease being spread through the water column. Ended up scrapping it, didn't see a difference and the disease ended up being a highly uncalibrated refractometer.

*Facepalm*

Update
 

Noticed last couple of days skimmer working better . Have it on a dry skim.
I'll check NO3 this weekend .