ID help please

I thought the purple was good? Of course I could be explaining it wrong so I'll attach a pic of it. It's only growing on the "base" rock it seems. I'm wondering if I should invest in more snails... It looks like algae but then it doesn't look like algae... I'm more concerned as to why 1 snail died and another appears to be dead. I'd hate to add more if they are in fact starving...

 

Looks like a dusting of Cyanobacteria. Coralline takes a few months and usually you see a florescent green form of Coralline take hold first.

Snails typically will not eat Diatoms, the Diatom bloom disappears once all the available silicates have been used up. Cyano, has many causes but in a new tank it can just be part of the evolution of the tank.

Try a different snail type next time, until you see film algae on the glass or the beginnings of Green Hair Algae you do not really need snails. Once you see the green film algae growing back on the surface of the glass everyday than that is a good time to add a couple snails.

You can also go to ReefCleaners.org regarding purchase and information about different member of your CUC.

As long as your water chemistry is good I would not worry much about the snails deaths for now.