He may be just fine, however, hippo's in general do hide. This in it's self is not alarming me. What does is the fact he wasn't quarantined. They are easily stressed, and are ick magnets. I quarantined my tangs--kole, hippo, and powder blue, in a 30 gallon, hyposalinity, for 3 weeks. Slowly raise the water back up to 1.024/25 and then release them into the main tank. They are fat, happy and well adjusted in my 125g. Yes, they can get stressed in a 20 gallon, but if you hypo the water to 1.009 you will ultimately drive the ick out, and the ick will die. Quarantining also allows you to make sure that they are eating well, before putting them in a tank that the others may be territorial about. I am not flaming you, I want you to succeed. Please try to catch him, and quarantine him.