They are hydroids. They live off of the free floating debris in the water. I had a very similar type growing in my tank. They covered almost every surface and grew next to my corals. Didn't bother the corals one bit. Eventually the hydroids died off. Replaced by some other dominant species of fauna or flora.
I'd just ignore them unless they are bothering your corals. Then just spot treat around your corals as needed. They will most likely be gone in a couple months if not sooner.
Edit: By spot treat, I mean scrub them off with a tooth brush.