Damnit... slip you bastard, you beat me to it. lol just kidding, good looking out...
your water is likely contaminated. Use some peroxide. and when you transplant, rinse all the excess of and repot in sterile soil. you may also be overwatering, but its unlikely. if you were, you would have already had this issue.
i use sunshine mix #4 to great sucsess. its a quick drain, so its really hard to overwater. plus its eazy to dry out