I'd guess an elm seedling. I'm in New England and find them in my yard pretty often. Not sure the species.

There is already a second one popping up in the next pot on the left.
Elms do get loads of seeds and there was at least one thread about the mess they caused when shedding seeds.

I would pull it. And those seedlings made it into those potts on their own accord most likely.

the free seeding elms in my area.. siberean .. are total and complete weed trees..

weak wooded.. prone to wind and ice breakage ..

a case where free .. does not make it of value..

in fact .... its worth what you pay for it.. nothing ...

not recommended for suburbia ...

maybe on your parents acreage??? where you can watch it over the next 50 years ... and then call me up and tell me i was right.. i will be 106 then.. i look forward to the call ... lol

really ed.. think about it... if it free seeded itself into an abandoned pot... that should tell you a lot ... its a PIA .... speaking of which ...

we just finished cottonwood seed season .. guess what will be in my pots in fall and next spring ... the other useless tree... lol

take care ...

ken