Experts help! What type of leaf/plant is this?

It seems to me a pokeweed seedling.

A weed is just a plant growing where you don't want it to.  If you don't want it, remove it, it's simple. For what it's worth it does look like a pokeweed.  The seeds may have arrived by being blown by the wind, dropped by birds, who knows?  Just pull 'em out.

It think it's a pokeweed seedling. The seeds are very commonly spread by birds and the seedlings seem to come up everywhere.

https://en.wikipedia.org/wiki/Phytolacca_americana

It might help if we knew what you planted and intended to grow.

take a sample to a garden supply store & ask them .................