Porcupine puffer with a whitish film

Greetings,
I have a porcupine puffer that is getting a whitish film on his back, it almost looks like he is molting. Additionally, he is periodically moving in jerky motions as if to try and shake the film off of his back, not to mention rubbing against the live rock. He otherwise seems fine, The closest thing I could find when searching the posts is that it could be slime disease. The local recommended Seachem's Metronidazole. we've had him for about 2 weeks, purchased him online along with a nasso, hippo and a dragonette, from Pet Solutions the other's look fine. Any help would be appreciated.
Thanks in advance

Here is a closeup. it is all over his back but it's most obvious on the brown spots on his back. I'll try for a better shot in the mean time.
Thanks again.