If he were mine and it was just a rubble pile, I'd prepare a sufficient vessel with sump water, shut down the system, move all the live rock over there, and plant a cut-to-fit piece of plastic across the middle, then start squeezing the space until I had the rascal in one place or the other. It once took me an hour to catch an amazing red-stripe 2" goby in an 8" pump chamber, but I got him, undamaged. I'm stubborn that way.