The best thing to do with nems is leave them alone unless they are in danger. Make sure he's in an area that is getting good lighting and as long as he's getting a little flow he should color up. Some say they feed, others say you should not feed a new nem until after its settled and stress free. I guess that's your call. If you do feed I suggest small chunks about the size of a dime. I see people throwing whole shrimp and hole silversides into nems that I would have problems digesting, lol. If your sand bed is compacted I might would suggest slightly digging out an area by him that he can slip his foot into. They can dig and it's crazy to see but the less energy he has to use to do that, the more energy he may have to begin coloring up. That's just my two cents. It seems most people set their rock structures up to be a visibly pleasant view from the front of the tank, which can result in the better flow being up front as well to suit all the corals, etc. that are in front. Well my haddoni lived in the back of a 93 cube for 2 plus years as happy as can because of this, lol. I got to see him periodically when he stretched out but despite hating where he ended up in the tank, I left him alone because he was thriving and he's now in my 180 and is still doing well.