Vermetid snails-a sessile snail that lives in a tube or spiral tube.They throw out a cobweb looking fishing line to capture food particles from the water column.Reef safe but can irratate and keep corals from opening.If you see a tube then these are what you have.
or
comb jelly-related to jellyfish except it sessile and really have no sting,the body is jelly/clearish and has long fine feathery web that also catches food particles.Reef safe but eventually dies off when the tank matures more.