I believe the problem was, that the five gallon pot was to deep. The soil at bottom of pot could never dry any, it was sopping wet all the time, breeding gunk.
So next time, use like a ten inch deep pot, and if your not sure potting soil is well draining, add equal parts, perlite to the potting soil.
Also ! Consider these questions.
Did pot have drain holes ?
Was excess water allowed to sit in pots drain pan ?
Was pot outside, where it would get direct sunlite ?
Did you allow top two inches of soil to dry out before watering ?
Was soil well draining ?