black markings

Hi guys, So a bit of context here - I've just moved in a watanabei angel pair to make a trio with my other female. To avoid any aggression after the acclimation box , I've covered the tank to be mostly dark for the last few days. However, on uncovering, I noticed this concerning mark on the...

Background: I have had three Chromis Veridis for about 3 months now in a 5 month old 29 Gallon tank that is very stable, although a bit on the dirty side nutrient-wise. The other two are showing no signs of stress or illness. No other tank members are showing any signs of stress or illness. All...