If you're referring to the long string thing, and if that string comes out of a hole in the rock and kinda reaches around, that's probably a spaghetti worm or something similar. Harmless critter that feels around for gunk to grab. Expect to see more strings.
The slime stuff is probably a sponge. Encrusting sponges are technically harmless, but some fast-growing species can overgrow corals.
Sorry for the delay and thanks to everyone for your comments!
I've uploaded the video to YouTube so you can understand what critter I'm referring to.
I'm not talking about the spaghetti warm, I'm talking about what the spaghetti warm is touching in the first few seconds of the video.
Thanks everyone! I did see it putting out the webby thing, so def a Vermetid worm or snail.
I'm actually trying to kill ick in this tank so I'm cracking up the heat there to 90+, I hope that it will get rid of the Vermetids as well.
Vermitids are snails, they just act like worms. You can cover it with liquid superglue (after removing the rock from the water) to kill it, or you can leave it alone. Vermitids aren't a problem unless you have tons of them irritating your corals, and it's small ones that tend to do that (rarely), not massive ones.
That heat might kill off everything else in the tank, not just the ich. Better to raise the temp slightly and just let the tank run fallow until the ich all dies off.