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This looks like Alligator Weed.
This link will help you.

https://aquaplant.tamu.edu/management-options/alligator-weed/

It appears to be Alternanthera brasiliana. They are not native to the US, and were only introduced to Florida. This is the only place that they can survive comfortably, outdoors.

They are quite invasive, and don't require any special care. Just don't let them get even close to freezing. They are strictly tropical.

Changing the soil once every year, and dividing them out can help keep it alive for longer. Feeding will not be necessary if you change the soil and divide every year, or two.

Outside of that... These invasive plants require very little other care.

It appears to be Alternanthera brasiliana or similar. Definitely an Alternanthera, though.

https://www.gardeningknowhow.com/ornamental/foliage/alternanthera/growing-alternanthera-plants.htm

Are they all green or just the new growth? When I look at photos of Plum Dandy, the new leaves look green and the older leaves are plum, as if they faded to plum.
Here are photos from a trial garden and the leaves seem to change colors. You might try giving yours more sun and see if that helps.

https://blogs.extension.msstate.edu/ornamentals/plant/tri17001/