Robert, I'm not an expert on growing mango from seed, but since you've gotten this far I would say let it go and see "if" it continues to sprout, you have nothing to lose at this point. Good luck!!

persoanlly, i would take it out,
and wash it off in hydrogen peroxide.

you can use the stuff from the store
look on the bottle and make sure of the percentage
it should be between 2 and 4 %
i would cut that in 1/4 at least

(dilute 1 part HP with 4 or 5 parts water)

use an old toothbrush, or something to gently rub the fungus off

let is sit in solution for 2-3 mins
rinse well...
re-plant