Help! Angelfish White Lip, Not Eating Well

Hi! I'm new to the forum, and I would appreciate some help.

I have a 120L planted aquarium with 4 angelfish.  The aquarium is in a steady state, started in February this year (had quite a few problems initially, but for about 2-3 months everything has been generally smooth).

Today I noticed one of them has developed a white, maybe fluffy irritation on its lower lip, and didn't seem to eat when I fed them afterwards (though he did come, and tried to rush another fish that was eating). The other 3 fishes aren't showing any similar sign.

Is this possibly an injury, or does it look more like some disease?

I did lose another fish  about 2 weeks ago, but he didn't show any physical signs, he was just smaller and was usually not let to eat by the other 4...

Water parameters:
T 28C
NH3/NH4 0
PH ~6.8
KH ~6 d
GH between 4-8 d
NO2 0
NO3 50

Welcome, @Simionescu

I echo Littlefish's advice.