Cute! It's perfectly fine. When a leaf gets old and can't photosynthesize anymore, the plant sheds it like this. As long as the leaves up top are still lush and green, it's a healthy tiny tree.

This looks like the fungaluglies at work, perhaps anthracnose. I'd remove the affected foliage with a sterile tool and dispose of the leaves in a sealed plastic bag or burn them. I'd also treat with a fungicide - a systemic would be most effective. Bayer's Disease Control for Roses/ Flowers/ Shrubs delivers the systemic 'tebuconazole' which is a very effective fungicide. As always, read/ follow directions and only spray outdoors.

Al

Thank you, tapla. Should I be worried about the other plants that were next to it?

Also, should I check the roots? Should I continue watering as usual, or could this be overwatering?

I cut that gray leaf and bought the product recommended, but now I'm wondering if this leaf is affected too. It looks dusty and different from the top leaves. Is it cause for concern?