Oh no! I sorry to hear it. Blotchy white patches usually mean Brooklynella (a parasite) or bacterial infection. Unfortunately, neither is particularly easy to treat. Don't be mistaken by Brook being a parasite because it does not respond to the average parasite treatment.
I am no pro at this and have only familiarized myself with symptoms so I know what to look out for and hopefully be quick to ID so I may (hopefully) treat quickly and correctly. This does not qualify me for offering treatment advice but I hope that narrowing down the possible issues is of some help to you. Please research Brook and bacterial infections in fish so you may know how to best move forward. With either of these ailments, you'll want to act quickly and I know online information can be contradictory and overwhelming, so my best advice is to look for advice online and written by a reefer who goes by Humblefish.
I agree with Stella that it looks like Brook which unfortunately is a quick killer and it will spread your other fish as well (although they may be better able to fight it off). This has some good info on how to identify and treat it: