Does anyone know what this is growing in my cactus pot

Bittercress? - a weed that is quite difficult to eradicate.

I think it is Hairy bittercress, Cardamine hirsuta. It's a pretty common weed, and naturally dies after it produces seeds. It is quite easy to just pull out of the soil, but it spreads very easily by seed. you can see all the seeds on those long narrow pods at the ends of the stems. Be sure to dispose of them in the garbage, not in the compost or thrown out the back door.

Thank you both!