he can go little awhile with out eating. I know at least a week. we've left for over a week and come back and our fish were fine. if he's hungry he'll eat the alge. some fish don't eat for the first few days. I'd contiue to try to give him a little but don't worry if he doesn't eat (for now). give him time. he's getting used to where he is.
or call the lfs and find what they fed him, maybe he's not used to the food you're giving him