I dont think that is a sign of light stress.. Light stress and sensitivity have a a set of symptoms, which I dont see in your pics.. What I do see, is the opposite of light stress.. Remember, fan leaves are basically solar panels.. When they start turning, its usually because they are looking for more light, or turning towards the light source. 50 inches away is quite a distance for led. Its even far for hps. Im not a master grower or anything, but that doesn't look like a symptom of light stress. I'd lower that light down to the normal recommended level for a bit and see how they respond.