We need all the info on your grow including: What type of medium; soil or hydro? What brand and type of soil? Indoors or outdoors? What strain? How old are the plants? What type of lights and how many watts? How far from the lights? What is your watering frequency and source of water? What, how much and when was it fed? NPK? What is the medium/runoff pH and PPM if in hydro? What are the temps and humidity in the room? What size pots? Any bugs? Look real close. Any other pertinent info? If any of these questions have already been answered please disregard as I cut and paste this for everyone. If there is leaf discoloration please TAKE PICTURES WITH ALL GROW LIGHTS OFF, ESPECIALLY HPS & LED's. Taking pictures and taking them with lights off, allows us to see what the leaf discoloration looks like.
Caterpillar poop I suspect, see a mite at half 12 o'clock, spray with Neem 1x time per month just to be safe
It's fly poo that happens on my plants all the time you can just scrape it off and I would suggest getting a insect repellant