As Joe said their “web” may irritate corals. I’ve kind that their presence is a sign if a healthy tank.
I’ve found them to be veru cool to watch at feeding time. If you drop some bits of frozen food or flake you can watch the slime strings come out. If y9u watch closely you’ll see them pull the strings back in the tube once some food gets stuck to it.
They are for sure a stationary snail like vermatid.
I think of them more of a sign as a very well fed tank. Their presence is almost directly proportional to the amount of particulate food added. According to Ron Shimek one of the few benefits of hermit crabs is they will help keep the number of these snails low.