The information you provided is not enough to come up with any smart conclusion, but I'll try:
Cleaner shrimp will not help. Being a hippo "specialist" I can tell you this fish is very easy to keep (if you care for your marine life correctly) and it's not prone to Marine Ich (white spots, etc) despite what you read on forums and in fish books. But there is a catch: this fish can be mental.
By mental I mean this fish is smart. Very much similar to a human being. It needs a house to sleep at night and when threatened. It's possible when reworking your rock you closed down the place where the tang was hiding/sleeping. Sometimes the fish will seem confused when you do that.
It's also possible you hurt the fish when moving the rock. With the information I have (fish size, how long you had the fish, what were you feeding it, any additions to the tank) it is not easy to diagnose. Chances are the new live rock you added contained some kind of parasite the fish is suffering from. Hard to tell. I would suggest not to bother him too much. You can drop the salinity a little bit in order to make it easier on the fish.
I have raised many of Blue Belly Hippos from 1/2 inch all the way up to show size and I never had any issues with these fish. If the marine life you add to your aquarium is properly quarantined/dipped and fed it will strive. Otherwise it will not. Make sure to feed all your tangs at least 3 different algae: purple (their favorite), green, and red. They have to eat twice a day. All food should be frozen. Fish do not process wheat.
Post some pictures of the fish if you can. Also explain your latest additions along with your quarantine process/dips, feeding cycle, and etc. Maybe that will help.