I scrape it all off... I do keep small little pebbles on top of the soil.. What kind of soil do you have?? You really need a bonsai soil.. wich is small pebbles or a near sand type of soil.. It binds the roots so it can't grow as fast..
Do you keep it inside or outside?? Mine I keep outside in the spring
and summer and inside during our winter... It could have to do with the soil used at the store...
Is there any way you can take a better picture of the soil?
Try this first .. scrape the mold off.. Get it some more sunlight and let it dry out a little more like I would say let it go 2 days with no water just to get rid of the mold... Not for Normal care.. do that and let me know how it goes ..