Looks like the start of mites. Check under the leaves for little dots that move. I had to stare at mine for a while and went nuclear and bought some No Pest strips and hung one up in my flower room. I know, I know. It's poison and you will probably get cancer from the offgasing but I think the stress from dealing with mites will cause me a heart attack first. I can't stand mites and it is one of the most common issues you will have. First, don't ever put plants outside and bring them back in. Once they sit outside they are contaminated with mites and whatever else nature has to offer. Second, prevent it all together by starting with foliage wash with Mammoth P or your favorite wash like what is suggested on here and do this every week or so until you are in mid bloom(always use food safe stuff anyways). Once you're in mid to late flower and no signs of pests you should be fine. Flower indoors with fast plants and the fastest methods. Start 12 on 12 off sooner and do more plants instead of larger plants. Larger plants require more love and lumens indoors and the pots must be able to hold them. It sucks root binding flowering plants in week 8 of 12.