Ah thats common. You See, those things you hold in your fingers are normally found on your plant - to find them in your fingers means you stressed the plant by ripping leaves off of it. No but in all seriousness :

There multiple deficiencies that i personally See in those leaves, Magnesium being one of them. Instead of going out and buying a whole lot of shit youll prolly Never need - try adjusting the ph of your water to around 6. the adjustment is to See if maybe your plant is locked out of nutrients.