Lack of light can cause bleaching as well. What light do you have on the main tank? If the sump is shallow, the 70 watt MH *might* be enough, but likely is not.
Assuming the main tank has stronger lighting, I'd put him in the main tank, in a lower area of light, and let him find the spot he wants. Do not move him around in the main tank unless he is about to go into an overlfow or powerhead. Just keep feeding him and let him recover, he know what he needs for light and flow.