Honeydew
Honeydew is a sugary excretion produced by certain sap-feeding insects, including many mealybugs and soft scales. On mango leaves and fruit it can feel sticky, attract ants, and provide a surface food source for sooty mold. The mold is a consequence of the honeydew rather than proof that a fungus is attacking the leaf tissue internally. Trace the deposit to the insects producing it, and consider ant activity and natural enemies instead of repeatedly washing the surface without addressing the cause.