mold or some sort of deficiency?

I was wondering if anyone can tell me if this is mold or not. Its off a big bushy plant so i am assuming thats the problem.

magnesium def. at first glance. but need more info..........

Definitely a a nutrient deficiency. Looks like you have a phosphorus or potassium deficiency. I'm leaning towards the potassium though.

Nutrient deficiency pictorial: http://www.420magazine.com/forums/problems-pests-disease-control/81275-cannabis-plant-pest-problem-solver-pictorial.html

Either way your plants are either HUNGRY or you need to flush your soil if you're using synthetic nutes.

its hydroton in flood and drain. I believe the yellowing of the leaf is from the plants flowering for the amount they have.

I was wondering more on the dry grey tips if i touch em they break apart the rest of the leaf is fine.