Does it look like this?
Vermetid gastropod or vermetid snail (stationary snail, looks a little like a tubeworm, sends out this spiderweb type netting to catch food).
If that's what you have ... Some say they're harmless, some say not. They are definitely a visual nuisance. I'm not so sure they're harmless, I find the webbing really irritates neighbouring SPS corals and eventually the corals recede a little. I try to let things be but every so often I have to pick out a few that get to be making a nuisance of themselves.