not too familiar with sea apples in particular but i do know that like their smaller relatives they can excrete a deadly toxin when they die..i lost an entire tank to that once.. i would keep an eye on him.. if he looks like he's starting to die i'd take him out. maybe someone else out there know's about the milky substance ..i don't think it's a problem ..but like i said i'm not sure.. what else do you have in the tank.. my trigger took a bite out of my cuke.. which is what killed it in turn killing the trigger... so careful cucumber though beneficial can be a pain.
good luck
Brian