Fish infected with Oodinium / Velvet become lethargic, but still continue to eat for a while. The yellow dusting of parasites are best observed under dim lighting. It's an awful disease, much worse than Ick. Fortunately not as common. Don't panic. As noted post some pics. Oodinium kills fairly quickly, so if your fish isn't declining chances are the problem is not Velvet. IMO
, copper based meds work better against Velvet.