I was more looking for an Id for the things pictured. Other corals and coralline are doing fine and growing. It’s just the zoas that are struggling. I’m thinking the snails may be the kind that eat zoas but I can’t tell.
The snails look like collonista snails, the second white lumpy thing might be colonial tunicates, and the little brown thing I believe is an isopod, maybe sphaeromatid isopod.
I was going to say collonista snails as well but the closer I look it's possible they could be juvenile sundial snails. Sundials will have a visible pointed operculum pointing up as the snail is in motion. If not than likely collinista-harmless. The white masses are colonial tunicates as another mentioned. The flatworms are likely red planaria which you will definitely want to get rid of. They don't eat coral but they multiply like rabbits and can actually smother coral with their numbers.
Can you pick up one of those snails, put it in a little cup or glass bowl, and take a well focused picture, particularly of the backside (opposite the spiral tip)?
Was gonna say harmless colonista as well but maybe the others are seeing something I don't, good quality picture of the other side would be able to clearly id sundial