Mold or salt???

Milk doesn't kill mildew. A 50/50 mix applied will only make your tent smell like rotten milk for weeks afterward, LOL. let the substrate dry out more, at least in vege.. then you can do the more frequent fertigation in flower -- will also have a more robust root system to take advantage of it. If it does occur it'll be much more reduced and much later in the grow that it will be inconsequential.

Oh yeah if it is say white mildew I believe a 50 50 mix of milk and water will kill it off.

I think it is both. Coco is high irrigation. Always wet and in my experiences, the more fluid passing through in a grow the better the plants tend to grow. My follow up question would be what is your run off ph, and ec or ppm? That information would tell you if it is salts.

Defiantly looks like mold to me. Doesn’t look to be effecting your grow but mold grows in damp and now airflow areas so try to increase your airflow to that. If you’re worried you could spray it with a mold killer but I have never had to do that. Hope this helps.

fungi I think. Might not be a bad mold could be an additive fungi they put in the nutrients. Salts tends to be dry and hard molds grow and are fuzzy and soft. Mold can also grow aorund salt deposits that's where it gets confuseing.

On top of your soil looks like mold, can't tell on the sides of the pot if it's mold or salt build up.