Fliptop,
That is vegetative Mango Malformation, caused by Fusarium fungus infection inside the wood and bark.
Cut off, cutting into the wood of the trunk. Sanitize wound with hydrogen peroxide. When that dries, spray wound with a Copper product, preferably as Copper Sulfate Pentahydrate.
Or cut off whole top of plant, a couple inches below the infection, and treat the same.
Sanitize clippers or blade very well afterwards.