Yah, I think it's something fungal. For several years, I lost 95% of my keitt crop to something that looked identical to that. This year, I used copper fairly heavily, and for the first time in 4 - 5 years, the entire crop is clean.
Mulch / organic material actually ties up copper and creates a copper deficiency, so I was also dealing with a heavy copper deficiency -- as evinced by gummosis and zero defense against fungal infection -- that nearly killed my own mallika.
We're trained to be afraid of copper, but it's actually a necessary micronutrient that is often lacking in organic soil.
There is a visible anthracnose streak on left side of the mango.