Thrips or fungus gnats. Usually checking under the leaves, down the stem, or watching the top layer of soil for activity will aid in identification. You can use sticky traps at the soil surface, plant sprays such as neem oil if in veg(not in flower). A good spray that hasn't led me astray is Lost Coast Plant Therapy. It's great for everything from powdery mildew to fungus gnats and thrips. It can be used all the way up until harvest as well!