Congrats on your new harlequin! Mine arrived from the same place today, and is behaving in a similar manor. Kinda hanging out under the rim near the top of the aquarium. He's by himself in QT, so aggression from tank mates is definitely not the issue. Probably just needs some time to settle in, as I suspect to be the case with yours as well. Does yours appear to be in good health? What are you keeping him with? I had actually ordered a medium Aussie harlequin tusk (3" - 4.5" as was stated on the website), but received one that is closer to 5.5" - 6". Looks like I'll be setting up my 300 sooner than I had originally planned. On a positive note, he has already eaten a few pieces of frozen shrimp and lobster from a feeding stick. I honestly wasn't expecting him to show any interest in food, but thought I'd give it a shot anyway. I do wish that I had tried frozen before trying to find live ghost shrimp at 3 different aquarium stores and striking out. I'll post some pics once settled in a bit more... would love to see some of yours as well.
-Ben