Missing fish?

I just added my first fish to a new 30g ecosystem that's been running for a few weeks. 2 days ago I added 3 small green chromis and some hermits in addition to the crab that hitched in with the live rock and survived cycling.
This morning though one chromis woundn't eat and is now mia. Can't find him anywhere. I even took a power head and blew around the rocks and in the holes and nothing.
Any Idea's? Is it necessary to find him?

 

Do you have a picture of the crab that hitched in with the live rock?? This may be your culprit. John

 

He's super shy, only out at night. He's dark red with a kind of lumpy back shell, 1" across. I'll take a shot tonight when he's out.
the chromis was looking a little sad since being introduced but the crab would prob take advantage wouldn't he.

 

Found the Chromis, No apparent injury's but he didn't survive. Everyone else is good though.
I could not get any good pictures of the crab but found out it was some kind of Eriphia. An evil little red eyed crab. One morning He had two Scarlet Hermits in his hole. It looked like he had them standing on the end of there shell so they couldn't crawl away, every time they stuck a leg out he jumped on them.
I found an easy method to catch him that worked in one day. I put a piece of shrimp in a cup and leaned it up against his rock, He went for it and couldn't get out. I put him in the fuge for now.
Tried to attach a picture of him but couldn't figure out how.