maybe not a mantis

[drock59]

about a week ago i was pokin around and looking at my tank. I saw what i thought was a little mantis(1-2"), milling around. He came out of a secluded area of the tank and beat the hell out of a barnicle. THen he went back in.
My question is, why hasnt this thing killed my fire shrimp, camel shrimp, stars, emerald crabs, porcelain crabs, etc, etc. I cant find one causalty of the little bugger. I would like to keep him but i dont think it would be wise.
Any good ways to catch him?

 

[brekurboy1]

The rest of the shrimp and stuff might be too big for him to prey on.

 

[drock59]

for now...