It could be the hermits, but I doubt its blue legs. I've never had much luck with Astreas. IMHO you're much better off with the slightly larger Trochus snails that are much hardier and can right themselves if they fall. Astreas can't right themselves, so they may die from that alone. Also, in my experience they are more susceptible to temp and salinity swings.
If you have a DSB I would check them for pyramid snails. These can migrate in on one and multiply quickly. They literally suck the blood of snails.
Margaritas are a cold water species...or I should say come from much cooler waters. That's your answer right there..
