Is he back in plain water now? (With conditioner, or course.)
It could be his slime coat. He might be shedding it, as a result of using the antibiotics....
Are you using API Stress Coat? If not, try using it, or another conditioner that has an additive to protect the slime coat.
I can see some gray in the picture.... but is it the same slime stuff that's on the plants?
Is he still in the QT tank? If so, do frequent water changes, to keep the water clean.
I haven't dealt with this before, but hopefully someone will see this and reply. If it doesn't clear up after another water change, or if it looks like it's getting worse, and if no one is replying, I would send a PM to someone like Sakura8 or Olympia. (Callistra hasn't been around recently.)