Bubble net

[markk]

I have a new tank and so far it contains 2 toadstool corals, live rock, and some snails/hermit crabs. One of the toadstools is doing great, all the polyps (I think that's what the tentacles are called?) are extended and it's looks healthy. The other ones polyps are barely peeking out every hour or so and there is what looks like some kinda of mucus net coming from a nearby pice of rock constantly blowing on it. Any ideas what this "net" is. I've tried to remove it but it keeps coming back any help would be appreciated.