Was zur Hölle ist das?

Dried up fertilizer??

To me, it looks like it MIGHT be some sort of fungus.... when you run your finger over it, does it wipe off? Spread? Not move (i.e., is it dry and hard?). I'd start researching some funguses and maybe think about applying some foliar fungus spray just to be sure. I'm also seeing a bit of calcium deficiency starting (the yellowing)... why don't you have a diary for this grow? That would help us help you! Good luck

Don't look like a disease, for me, it looks more like some drops of liquid concrete or something like that