Algae blennies are micro-algae eaters by nature.. diatoms, glass algae, etc. While some will eat seaweed selects or "tang heaven" type stuff, many will not, and most never eat prepared food at all, although some will. It could be an internal disorder as mentioned, or it could be drug catch as this is common with this type of fish. The majority of indo algae blennies are doomed before you even get them. Having your LFS feed one isn't a good indicator.. what you should do is watch it at the store.. if it's picking all over the glass and decorations, you have a better chance of getting it to live off the micro algae in your aquarium. If it's just sitting there doing nothing at the LFS, don't buy it.