What is going on here ??

[Sam1969]

Recently I have had some issues with my tank chemistry but I think I have it all fixed now .. however there have been some weird things happening lately. For starters .. I have 1 blue legged crab ... or so I thought ... I put it in about a month ago, then today I noticed it out of its shell dead on a rock today .. so I removed it ... then after throwing in the garbage and returning to the tank I picked up the shell he was in to move it down on the sand with the other shells when all of a sudden another blue legged crab appeared from the shell... now I notice alot of small ( 1 or 2 mm) white worm like critters on the glass of my tank some are larger and have curled up but there are alot of them and they move rapidly does anyone know what these are.