As already pointed out "red berry" bush can be a number of things with very wide ranges of care, from full outdoor plants (in Cleveland) to full indoor plants, to plants that fall in-between, i.e. cold room plants. Yews, Cotoneasters, Fukien Tea, all produce red berries and those are only one of each of the examples above, there are many more.
I'm curious though, we do you think it needs to be repotted? If it has to do with glued on pebbles, yes, you're probably right. Otherwise, it's better to wait until growth starts in abundance (Spring).
Watering and misting are two diffrent things. Misting is only done to raise humidity levels. It provides very little to no water to the soil. In fact, it can make the soil's surface appear
to be a lot more moist than the rest of the soil actually is. Until you get to know the tree better, how much water it needs and how often, you might want to lay it on its side when misting, avoiding the soil. As far as watering goes, most trees should not be left to dry completely between waterings. Some, as well, do not like to be allowed to dry at all. Most of the species we keep, however, fall somewhere in between the two extremes. A chopstick or toothpick stuck into the soil might be a better guide than the soil's surface (which, thanks to forced air, may be stripped of moisture much more rapidly than the rest of the soil). I know these are a lot of variables to consider, but you will do well to always ask yourself, "Are your SURE?", before doing anything. This tends to make you MAKE sure and avoid assumptions.
On the mold/mildew: Are you sure that this is what you are seeing? Are you sure it is not calcium (lime) build up from your watering? Alternatively, moss is prevalent in bonsai culture, from the mass produced "mallsai" to master's emperor-sized trees. Spores make their way into virtually all pots. Moss may not do well in one pot (in the same location, same watering and humidity levels) where it does in another. You can even add spores to some only to have them never grow, where pots to which you have never added spores will show lush growth. My point here being that even if the bonsai did not have moss when acquired, it can develop "spontaneously". Sometimes, some moss species start by looking like green mold, or algae even. It's another possibility, other than mold/mildew, for what you are seeing.
It's hard to say what you can do to get rid of a thing when we do not know what that thing is for sure. For mold/mildew, backing off on watering is the most likely course to be taken since over watering is the most common reason for their occurence in the first place. For moss, a medium to hard toothbrush works very well. For calcium, distilled water, possibly with a few drops of vinegar, wiped carefully with a q-tip works well. Any time you use something that probably should not get into the soil (like soap or vinegar), it's prudent not to allow this to happen. Again, laying the tree on its side is the easiest way of avoiding soil contact. If the soil is loose, which it should be, you'll want to secure it in place somehow, or tip the pot only slightly (not past 45Ã‚Âº).