Hmmm I too think you might have a mantis shrimp or crab problem. I have a good size mantis living in my 300g tank, he was hiding in some live rock I put in the tank last year. I tried to get him out, but with such a large amount of live rock in the tank I would have to break the whole thing down to try to get him out. He has a tunnel built underneath a whole pile. I learned real quick that he will take out any small fish he can get to. So now we only have big fish in that tank. Oh and he loves snails
If you hear a clicking noise coming from your tank, you've got one. The hair algae can be taken care of by either scrubbing the rock or get a tang who will eat it.