Well since I really really love Keitt mangos, I guess I should try to help. However, Har would be a better choice to answer this.
My guess is you have not given your mango tree enough copper. I also see a whole lot of vegetation which might imply (not definitely) too much water in the soil around the roots, so also try to provide an area to allow water to drain away from the tree by way of a trench etc.
Kocide, Nordox, etc. would be some copper products.
I have only tried nordox for my Keitt tree problems and still have a bag from many years ago as I believe it is a tablespoon per gallon of water. I would spray the entire tree probably once every 10 days for the next few months and it will probably also help decrease the lichen but you could scrape it off.
You can also use oil & might add an insecticide if applying to the bark for the lichen.
I would also fertilize it heavily after removing some of the vegetation. If a plant grows fast enough, disease seems to just vanish away with the rapid growth.
As a final note, disregard all of this if Har etc. give their opinion.