I have 3 whites tree frogs and one of them is always brown and the others green. Occasionally the two green ones will turn a little brown but it's completely safe and the brow one is actually a male and currently breeding. Also like you said your frog keeps hitting its head against the bowl. I recommend feeding your frog in the tank instead of in a bowl and it will also help to tong feed your frog if it can't hunt as well now. I hope this helps your worrying. Everyone worries about their frogs.