Sea Lettuce (Ulva) is enjoyed by just about every herbivore on the planet, so yes, it might get eaten. I don't know what Pom Pom is. Full size Ceriths may have a hard time getting to your macros, if they are floating/tumbling in your fuge.
I'm curious why you'd add snails to your fuge, rather than your display. Bottom line, I'd move the snails to the display, and leave your new macros unmolested in the refugium.