Either uronema or a bacterial infection (Vibrio).
If you can take a skin scrape (see here), and examine that under a microscope, we have slides of what uronema looks like:
BIG THANKS to Dr. Charlie Gregory for providing these slides/videos. His website can be found here: https://www.aquaveterinarian.com Marine Ich (Cryptocaryon irritans) Marine Velvet Disease (Amyloodinium ocellatum) Brooklynella hostilis Uronema marinum EDIT: Apparently sometimes Velvet...humble.fish
I have this one, it works well enough for disease and algae ID. Get this to go with it, it makes taking pics and video much easier: https://www.amazon.com/dp/B07JW9KHZ2/ref=cm_sw_r_cp_apip_ZBcIw2qqWO5s2
Good basic scope: https://amazon.com/gp/product/B00AM5XB5O/ref=ox_sc_saved_title_4?smid=ATVPDKIKX0DER&psc=1
A bit better with USB connectivity: https://amazon.com/gp/product/B00ANPEZP8/ref=ox_sc_saved_title_7?smid=A35BV4NHHC5J5C&psc=1
Excellent scope for the price: https://amazon.com/gp/product/B0094JTZOU/ref=ox_sc_saved_title_7?smid=A35BV4NHHC5J5C&psc=1
I was using peroxide to treat suspected velvet in the DTs. I’ve been peroxide dosing for months now, and have successfully added a number of fish to my tanks since. I’ve now tapered off the dosing in two tanks, and will be tapering off the other two here in the next month. After that, we’ll see if anything comes back once the UV bulbs start to die. So far so good, though.