Aipastia will latch on and grow on anything in the tank. I have had them on the actual polyps of my palys and every piece of plumbing as well. I've also had them on alive moving snails and hermits but they seem to detach sooner than later in those instances. lol
Is the shell an alive critter? if not take the whole thing out and call it a day, you DO NOT want them to spread cause it's the worst thing in my couple years of experience that can happen. Green algae and cyano aint got nothing on those freaking aipastias.