Looking for info on my corals

[noxthedragon1236]

So I have a couple of questions. One is what is happening to this blasto is it the digested food I just feed it? Next is what could this white stuff be on the rock. And is this a clam of some sort under the rock. Also lately my zoas haven't been opening much and have been regressing a bit. The next and last question is this is a 13.5 gallon tank I bought 2 blue leg hermit crabs. And there happened to be a 3rd that was hiding in the used tank that I missed when setting it up. I'm wondering if the 3 hermit crabs are too many for this size tank as I also have a emerald crab and a peppermint shrimp. Any idea what the mistery hermit crab is he doesnt have blue legs. Today I found them ganging up on a snail and killing it. Wondering what to do.

 

[Mr. Mojo Rising]

blasto looks like its pooping, corals do that sometimes, its fine.
The white stuff looks like a sponge, but wait for other confirmations.
Need water parameters, type of lighting and flow and tank age info to help with the zoa's.
3 hermits in your tank is fine.
They might be hungry if they are attacking the snail, make sure to feed them. They might also want a larger shell, so find some slightly larger empty shells from somewhere to add on the sandbed.