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Black Locust Trees can be an issue.
The trees are used infrequently as landscape trees for a few reasons. First, although the wood is strong, branches are brittle and prone to breakage in high winds. The trees also have thorns. Black locust trees are very susceptible to attacks by the locust borer, which often prove fatal. The trees are more often used to control soil erosion or as a timber tree.

If you didn't poison the cut stump immediately with a systemic herbicide that kills the entire root system, then the tree has gone into survival mode. The growth popping up in your yard is probably suckers off of the lateral roots that are still alive.

At this point there is no easy way to eradicate the suckers. you have to chase them weekly as they appear and cut or spray them, probably for a couple or three years until they don't come up anymore. Don't let them grow at all or they will photosynthesize and feed the roots.

The only other way is to dig out all the lateral roots.

No photos came through so can only provide best guess. It could be natural color of black locust, or it could be sooty mold growing on honeydew secretions from an aphid infestation. The mold is not harmful, but the aphids are sucking insects that can weaken the tree.

Some type of physiological stress like water deficit could be the cause.

It does appear to be Gleditsia, but not the Honey locust. Probably a more ornamental cultivar with a low thorn count.

This article will offer more information: https://www.gardeningknowhow.com/ornamental/trees/locust/locust-tree-information.htm

Sure, you can grow trees in containers. This one will need a very large one, but it can be done.

Here is an article that will help: https://www.gardeningknowhow.com/special/containers/how-to-grow-container-trees.htm