Any idea what this is? Seek says it's in the genus Erythroxylum, but it can't get any more specific than that.

Automatic identification via PlantNet summary

Most likely match: Reynosia septentrionalis Urb.

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It's definitely not a Reynosia, they have opposite leaf arrangements while your plant has an alternate leaf arrangement. Good luck!
- Good to know. Are there any other leaf characteristics to look for when identifying plants? I was just thinking of shape and color, but there must be a lot more.

Unfortunately, the Caribbean is a large and very diverse region which is underrepresented among the community here and probably in training data for plant apps as well.
If you can come back when it flowers or fruits that would make it easier. Or sharing a more specific location.
Erythroxylon rotundifolium, urbanii, carthagenense, havanense, and especially brevipes all look superficially similar but I’m not familiar with this genus. They all have glaucous leaf undersides so consider adding a photo of that feature to help verify.
If you are really committed to IDing the plant, you may need someone with local expertise.