What they are doing is catching small particles to eat from the water column and will reel in the nets (cobwebs) and eat what they have caught.
Nothing eats them. You may have a number of them because there is plenty of food for a large population. You might want to cut back on how much you are feeding so the fish can polish off the food in just a couple of minutes and there isn't much left for the snails to eat.
Thanks Kathy,great article,no doubt I have vermited snails and the webby stuff I'm seeing are the mucous feeding strands,now that I know what they are it's pretty cool,a little biodiversity as long as they dont get out of hand,lol.