Not mentioning anything about the tank size-
As long as he'd still eating, I wouldn't worry too much about it. Juvenile Hippo tangs are pretty much strictly zooplanktivores, and it's not until they reach adulthood that the algae is so important. Not saying you should stop trying, but it's natural for him to want to eat brine, cyclops, etc. while still young (under 3" or so)