crab hunting

[Graham_f]

hello,
I'm new around here so take it easy on me

I've had my first tank up and running for a few months and so far everything has been going pretty smoothly so far, aside from a small issue with some green star polyps that decided to die...and start a fun algae bloom. so much for the one coral that "anybody" can grow
while cleaning up the mess i spotted a small crab and quickly lost him into my rock. At first i was quite excited about the new find but after a quick google search i found out he was not welcome in my tank. ive only seen him twice in three months but its grown very fast. prob about 1.25" across now and was last seen picking the insides out of my late starfish

the time has come, he has got to go now.
I've tried the DIY bottle trap with various baits but he wont leave the rock.
any ideas? my live rock setup doesn't allow me to take rocks out without much frustration.

thanks