I got this beautiful blue CT about a month ago who was a rescue-buy from a LPS in a cup and quite neglected. Put him in a nice 5gal home with clean water and his color improved and fin issues improved. About a wk after having him I noticed whitish area over his very dark face. Then noticed same under his jaw. Thinking cottonmouth or columnaris. Water changes for a week. No change. AQ salts and water changes. No change. Cairo still very active, voracious eater. Fungal treatment. No change. Erythromycin treatment. No change. Today on closer observation, I see a similar coloring over his back. I think this is just his coloring coming out! It's actually a beautiful irradescence over a dark face, happens to be in the same distribution as cottonmouth.
Thank you sweet Cairo for your tolerance and patience!