My hippo wouldn't eat from the clip either for a while... even while watching the other fish eat from it (yellow rim, naso, starry, cleaner wrass, and the cow fish). I lessened the feeding of other foods until I saw him nibbling from it hesitantly.

Now they all eat from it. I don't put it in as often as I should though. The cowfish goes in there and makes such a mess I'm worried the other fish aren't getting it all before it becomes lost in the rock.