that does look like mildew....make up some water with hydrogen peroxide and it will kill that, if you're using the 3% stuff from the drugstore, use about 20% h2o2 to 80% water.
that may or may not have come i nthe soil, it's usually found in grow rooms that aren't very clean, have too high RH, and poor ventilation....does any of that apply to you? if not, it might be the soil...if so, i'd just get rid of it
if any of that does apply to you...FIX IT