Clown sick

[HarleyC]

Small clown is looking sick - has some white on him, but I don't think it's ICH, looks more like dusting from sand. Fins are looking frayed and he is swimming up against the back corner. He wouldn't eat too
The rest of fish and inverts all look okay.
I purchased the established reef tank and we set it up here 3 weeks ago.
He looked okay until yesterday, when I did a 25% water change because of high Nitrates - coincidence?
Should I remove him in case he infects anyone else, or see if he recovers?
Thanks,