Sasha,
As already mentioned above, it looks like cyno. Your tank is still maturing and it's growing in that area of your sand bed because that's were there is the least amount of flow and or the most available light reflected from the sand.
With a young system like yours there are a few cycles to go through, not just the water has to cycle but the sand bed needs to seed and cycle itself and so will your live rock. This is the hardest time for reefer's and fish keepers alike due to the fact that sometimes these cycles can last forever if properly fueled. If you follow the advice of others, read a few books, many more threads and keep your tank maintenance on schedule then you should pull through this and other cycles without a too much worry.
Also during this stage you need to use extra care in many ways like: not overfeeding, keeping filters clean, water changes on time, etc... otherwise if you don't then your simply fueling the cycle and more undesirable events will happen.
You should also research everything before you place it in your tank, this is just common sense. Buying an anemone for your clowns was a nice thought, however your tank is still very young and anemones can be very sensitive to change so take care. It looks like a brown long tentacle which is hardier than some others. I had one for about 2 years until it outgrew my tank which both my pink skunk clowns loved. As mentioned tho clowns are very selective by nature when it comes to choosing a host so even if you have the right anemone that your clowns usually host to by nature doesn't mean this action will take place in your tank. After I traded in that anemone my clowns were fine by taking up host in a large frogspawn coral as a surrogate host.
One final comment if I may blab on and on, is regarding that grape macro algae that you have. IMO it's not the best thing to place in your tank, it would be better in a sump or refugium as the cyno cycle that your having will help fuel it's growth which if left unattended you might end up with a tank full of it. Instead of using plants and algae to help remove nutrients from you tank at this time, I would suggest increased water changes-that is if your using properly treated RO/D water and not just treated tap water.
Good luck and sorry if I seem rather blunt, but you can save yourself a whole lot of worries in this hobby by being aware of what can happen before it actually does
