I have had many seedlings spend their first few years in a pot about that size. The only thing you want to do is protect the roots a bit. I usually put the pot in a protected corner outside during the colder months. The white splotches could indeed be mold, if the sun ever does shine again, it should disappear. They do have fungicides but with a plant that small, I would be cautious of using it. If the sun does shine and it does not seem to get better, you could try a systemic made for houseplants. That should be gentle enough.
You will know when your tree is getting root bound as it will stop growing. At that point, you either need a bigger pot or a real home for it.
I have had success with Chestnut trees, oaks and maples. Enjoy.
diene

Since it's a volunteer, we can be reasonably sure it's hardy to your zone, but that doesn't mean it will survive in a container w/o some degree of protection. I would bury the container in the ground, out of wind and sun, and mulch it. It should then over-winter just fine - as long as you keep the rodents away.

It's very unlikely the white blotches that look like bird droppings are mold, and very likely they ARE {{gwi:2103049}}.

Al

What can be done to get rid of cottony maple scale?

Generally, it's not treated. Late spring (before bud-break) applications of hort oil are fairly effective. There are also a number of other chemical insecticides that can be applied in early summer when the crawlers are out. You don't tell us where you live, so it's hard to guess when that might be for you. (you should include your USDA zone and state or a large city you're near in your user info).

Al