questions about invertebrates

I'm assuming that this is a bad sign. I've had a margarita snail for about a month, and I noticed that in the past few days it appears the coralline growth and patterning of its shell is flaking away to reveal a pearlescent white shell. I've only used pre-made Saltwater and distilled water...

As title states, I'm looking for a reef safe sea urchin for GHA in my reefer 350. Right now all I have are some softies as well as some fish (clowns, cardinalfish, damselfish). Looking to see what my best option(s) would be.. I've come across pincushion, tuxedo, and rock boring.. thanks!

Can can add a invertebrate straight to my display tank? No quarantine