I wrote this for someone here having this issue on another thread but to make it easier for others needing this info I have copied it over to here.

I hope this doesn't come to late and maybe it's pointless if you can't get your hands on some Neem oil. It has always successfully killed that white mold that grows on unsterile soil. It doesn't take much for a tiny bit of something to rot and start this mold. Neem is good for insects too- get your hands on some of this stuff even if the following solutions work for you now. It'll be good in the garden come spring and especially for killing and preventing hatching of insects. Mix a teaspoon in an old clean fantastic bottle (what ever you have that size) with 4 drops of dish detergent and fill the rest up with water. Shake well and in between uses. Squirt over soil top.

1 part bleach in 10 parts water. Squirt over soil top lightly, you can always resquirt if necessary. I haven't tried this myself but I hear it works without hurting your plant.

One simple treatment can help keep your house plants mold free. Taheboo tea (this can be purchased at most health food stores - I've personally never heard of it...lol). Just adding this tea to your house plant water in a mild solution is enough to do the trick. I don't know if this works I just read it elsewhere while looking for a chamomile tea tactic. Chamomile tea is used to prevent dampening off but it didn't work for me, could be the tea was just too old or not strong enough.

If you haven't any other option at this time... A desperate thing to do is... removing the plant and washing all the dirt away. You will lose a lot of the roots as they are very fine when they first start out but it sounds like you could manage to save a few. It's not going to kill the plant if you have to re-root. Cut away its pot or slipping the dirt ball out and never pull or hold the stalk. Once the dirt is clear, squeeze the along the branch/stalk and feel for any hollowness or if you can squeeze into it. Also look along the branch for black - I'm sure you know this is rot. Any soft part would have to be removed. Soap and water the rest. The bleaching of all pots as well has to be done or get some new ones.