That looks like bryposis. Some reefers have had good luck with Kent Tech M. The magnesium additive has some sort of algecide and when Mag is elevated to high level it will kill bryposis. I did it in my tank and it worked well. There are some threads around about this.
If it was my tank I would do a lot of water changes and remove all my sand. I found in my nano I mas more successful going barebottom and siphoning out the fish poop weekly.
Also, I only see one fish but a low bioload is your friend in a nano.
Another option you have is to cook the rocks.
I would also add to the cleanup crew. I would caution you though, that once you get the nutrients under control you might not need a large cleanup crew. In my barebottom 34 gallon Solana I only needed 3 snails and 5 hermit crabs to keep all the alge in check. I guess any excess you can put in the 55 gallon.
Hope this helps, i'm sure you were already thinking the same things. Good luck!
