I think The Alien evolved from aiptasia

[Mephistefales]

Lived for three days in a nearly dry bag of chaeto (I'm *done* w/ sight-unseen internet livestock now). I removed a bunch to a plastic cup and filled it w/ hot water from the tap. At least 6 of the little boogers lived for about 10 minutes before finally departing for what ever hell they belong in.

[CoryDude]

I've had them show up on items from reputable dealers too. I restarted my tank from dry rock and was very thorough when adding new livestock to the tank and I still got them. If they become a problem, I'd recommend berghia nudibranchs. I added five to my tank last week. I've already noticed that a few of the nasty buggers have started to disappear.

[Mephistefales]

I think I've heard about those before. Are they supposed to be better than peppermint shrimp? Also, would they be doomed if they slipped onto one of my 4 koralias?

[CoryDude]

Never tried the pep shrimp. I kept hearing they are hit or miss. The b. nudis will not get sucked up into a powerhead. They attach themselves pretty well to just about anything. If you have a wrasse that likes to pick at rocks (i.e. 6-line or a melanarus) then the nudis may not be a good idea.