Was this a vollunteer? Did you collect seeds or soil from any wet marshy areas? Did you collect any unknown Apiaceae seeds? It looks like a Ptilimnium species, Possibly P. capillaceum? The keys. The flower looks bunched up and not unraveled? Ptilimnium capillaceum is the most common Ptilimnium species in Dallas.

What does it smell like if you crush a leaf?

Ptilimnium also smells like dill.

Hmm, Ptilimnium capillaceum (Threadleaf Mockbishopweed) does look like it!

How it got in my planter...I have no idea, though? As I don't recall transplanting anything in there (just growing seeds).

And it does have a dill-ish smell when I smush its leaves...