First grow, newbie grower!

give it a gentle rub.. does it come off? if powdery, yes it is. If more of a calcification or mineral type materia, it's just dried up water. keep the leaves dry if you drip on them, especially nute-water. if they've been moist lately or you foliar spray, stop doing that unless there is a specific reason to do so.. not just a daily habit. all that does is create a constant and welcoming environment for pathogenic microbial growth.