Hey man I've had exactly the same as you, it is a nutrient buildup, but it's nothing to worry about, yet... it's the first signs there might be something up though. Based on what you're putting in there, I would suggest following the nute, plain water, plain water rythm. Beyond that consider adding some micro nutrients. Why not use molasses for that : it's cheap and it's bio ! Just add a bit of that on plain water days