They are just old lower leaves. Ready to drop

If only those leaves are grey is not a problem....the old leaves fall. Look the new growth to understand the health of your mango

Bad pic, but it looks like you have a sooty mold problem Spray with Neem oil and a copper based fungicide. Is the plant partially shaded? This could contribute to the problem. Also, sooty mold thrives on the residue from insect damage so the Neem oil should help in a few weeks, but you might spray with some Malathion to knock out anything that is sipping on the leaves. Sooty mold won't harm the leaves in itself, but it will hurt the photosynthesis and is indicative of a bigger problem.