1st grow cookies gelato RQS

fed all plants 700-800 ml of nute water with the mixture shown above ppm of water before nutes is 30-40 ppm of water with nutes was 500-550 ppm of run off water was 1100-1200 pH of nute water was 6.4 pH of run off water was around 5 next watering will be with just pH balanced water. any thoughts on what all the whites spots are from? I thought it could be some sort of deficiency since I haven't fed them in a while I'm also seeing white specks on some of the leaves, not all of the leaves, just a select few leaves, could it be early stages of powdery mildew? please give me your thoughts before I start spraying my plants with stuff and what about the missing parts of the leaves? kind looks like pests but I don't see any other sign of pests also some of the bottom and oldest leaves are yellowing and withering away, is this okay? is it a problem or normal, the leaves in question get almost no lights, should i just cut them off now or let the plant recycle them?