He has been trying to scratch his itch on a piece of live rock.
Ahhh, I didn't realize he was scratching himself. It still could be bacterial or fungal, but with the scratching, the spot on him, and the fact that it's a new fish, it could be ich.
I would go with what Drobes said. It's hard to recommend something when it's not fully known what it is. Can you get pictures of his white spot?
People believe the garlic's useful because it attracts a lot of fish to eat and continue eating while also boosting their immune system to help fight off certain diseases more easily.
If it is ich then I definitely can't recommend treating your main tank. A separate set-up will allow you to dose medicine and use hyposalinity if you choose, without affecting your inverts.
If it is bacterial/fungal, I would still recommend the Melafix- preferably in another set-up, but it's still okay for your main tank and will not harm your inverts.
http://www.aquaessentials.co.uk/index.php?main_page=product_info&products_id=472
There are a lot of threads on here where others have used Melafix with success (including myself).
I'm worried it might be ich though, which would be a completely different course of action.
Either way, I would still feed with garlic/extract, even on a normal basis. Can you describe the spot if you can't get a picture?