Oh, new friend... if only it were that simple, that only the seeds gardeners put on the ground were those that sprout. Seeds blow around in the wind and are moved in various ways by animals. I don't say this to insist my suggestion is the name that matches your plant, but so you will know to expect a few plants to have come from seeds other than those that were in your package. Every garden has weeds to pull.
Wildflower mixes are frustrating that way because few gardeners would recognize all of the seedlings listed on one of those packages, and a few of the plants, when they finally bloom, won't be keepers. BUT, that's part of the excitement, isn't it?! Seeing the bloom and deciding if you like it or not.
Hopefully your plant is something other than my suggestion. If it is, it will soon get fuzzy at the top of the tall parts, with ripening seeds, ready to fall or float away in the wind and sprout wherever they can. If you notice increasing fuzziness, that means what you've already seen was the bloom, and the seeds have formed. If that's what you see, I would encourage you to pull it. I never see butterflies visiting these, my yard is FULLLLLLLL of them, and it's not native in the US. These plants are medium-easy to pull if you grab all of the stems and the ground isn't too dry. Pull gently but increasingly, hoping to pull up the taproot (so it can't grow back.)
