NEED HELP STAT!

Hey all, I am new to this whole site- but am looking for some helpp.

I woke up this morning to find that my new emerald crab had eaten a $30 coral frag- so I traded him at my LFS for a couple more snails.

I tossed the bag in the tank to acclimate and noticed I had a little hitchhiker- before adding him to my tank, I'd like to figure out what it is make sure it isn't a toxic nutibranch that is gonna kill everything or something like that.

He is white/clear- ...