No crab - save for porcelain crabs, anemone crabs, and the commensal crabs that live on coral - is 100% reef safe. At some point if he gets bigger he will eat snails and fish, though most likely leave coral alone.
Don't bother trying to catch him with a net - set up some sort of trap. The cheapest/easiest involves a glass or tupperware container and sinking pellets (which are good because fish ignore them. Make sure the crab has a way into the trap, but no ramp to get out.