oh this sounds so familiar.....
let's see....green brittle starfish in tank.....7 new baby chromis added to tank...1 chromis MIA every morning for every other day for the next week....oh yeah, it was an all-u-can-eat chromis buffet for my starfish at night!
my starfish never came out when it was time to feed the other fish, perhaps i would get a glimpse of an outstretched leg through the rocks, feeling around for food, but that's about it. never saw the damn sucker come out.
THEN! i waited til it was dark...past midnight. i turned on an ambient room light and THERE HE WAS!! Out and about, stretched as tall as he could, flat up against the back of the tank, writhing and twisting to get at a scared chromis only an inch or 2 away from his leg!!!!!!!! OH I GOT YOU NOW!....he went to the lfs the next day.
my advice, catch him at night. he'll be rolling about, but they are fast suckers. good luck!