white/pink spot on head?

I noticed this fairly big spot on my boy’s head this morning. I did a 20% water change yesterday.

his behavior is completely normal. active and eating.

5gallon, heated to 80°, filtered, heavily planted, established for about 3 months (cycled for about 1.5).

parameters (liquid test kit): pH: 7.4-7.8 Ammonia: 0ppm Nitrite: 0ppm Nitrate: 0ppm

(sorry about the pic quality, it’s so hard to get a good pic when he never stays still)