I used to lay the fillet skin side up (skin removed) and with the fillet knife cut
from the top and bottom towards the middle, shaving that red portion out. If you have ever had a salmon fillet it is the same as the thin layer of meat next to the skin-cooks up grayish/tan and mushy. I have noticed catfish get
fatty like that when they get above a few lbs also. Didn't mean to butt in the post but have cleaned many whites.
Jeff