White mold in dirt

I repotted 2 or 3 days ago and today i noticed some white mold where i had added new dirt. This is the same dirt i used to veg for 2 weeks for two plants and so far there had been no problem. The soil is moist but not soaked, i have removed about the top 2 inches of dirt and wont add any new yet unill i see if it grows again. Anybody know what this might be, what causes it and if it i harmfull. Im sure its not good for the plant but maybe this is something common that usually goes away.

This is the area of where the mold is


Close up

I started flowering 2 days ago and i removed a fan inside because a deat issue was resolved, but i think that with less light now, lower temps, less air circulation (still have 2 PC fans blowing air out) and dirt being wet longer because of repotting to bigger pots that this might be the reason for mold ot be showing now. I will add the fan again so that the warm air gets to the bottom, the dirt again, and make sure not to water untill the surface is dry. Also to add info, i am using fairly good soil from the store.

But still i would like to know if this has happened to you and if it continued to be a problem later on.

the soil is too moist and the air circulation is not enough. i had the same issues with my grow box. you can kill most of the mold with hydrogen peroxide diluted in water but if you keep doing the same thing the mold will come back. the peroxide wont get rid of all the spores.

dont water until it dries out a bit. make sure the top soil is not too moist when it goes in the dark and get that other fan in there to provide extra circulation. proper air movement will keep the mold from forming even if the soil is wet and in the the dark.

at this stage its not harmful to the plants but it will be if left unchecked.

thanks thats a lot of help. +rep