I have one as well. I was told it was a Moreton bay lobster. (they look very similar) I posted previously about these & did not get much feedback.
Mine has more orange on its body, but the same markings. It is now about 5" long & just recently molted.
It has not harmed my emeralds or any fish.
It started eating my snails at a rapid pace (moved them to the refugium).
It does like krill & silversides as well & will eat daily. It started hiding a few weeks ago & not eating, I thought it may be sick or ready to molt. This week I saw it on the bottom of the tank upside down & figured it died. When I moved a rock to pick it up, it came walking out past his old shell.
Lonnie