Green stuff?

[ScaryBonsai19]

What's all this green stuff growing near my Chinese Elm, can I remove it?

 

[rockm]

That is moss and algae. You can remove it, BUT it is a symptom of issues with the roots of the tree and the soil that the tree is in. Moss needs a constant, reliable supply of water underneath it to survive. That means the soil is staying too wet and/or not draining well. This will get worse if not attended to. Repot next spring, be careful with the water. Water when the tree needs it not on a schedule.

 

[Paradox]

Yes, remove it before it spreads across the whole pot. Just do it carefully because sometimes there are tree roots right underneath it.

 

[Shibui]

The flat ones look like liverwort. It will indeed spread rapidly and can cover the entire soil surface. Also moss and harder to see but probably algae as well as a few grassy weeds.
Most of those are definitely a sign of too much water.