Not sure. It looks like a deficiency except that its not happening very evenly. I'd head over to the GrowWeedEasy site and check out their plant problem guide. Very, very useful.I recently noticed an issue in one of my 2 indoor Blue Dream plants. This just started about 4 days ago and it seems to be getting worse moderately fast. It's only happening on 3-4 leaves but seems to be spreading slowly as well.