Mystery weed

There's a weed growing out of the middle of my cactus. I tried to pull it but it was just too strong and I just ended up leaving it alone.
At first it looked like a grape vine. I'd had a grape vine at the old house but it died and I just planted something else in the pot. But, it's not a grape vine. It looks more like a tree, like a fruit tree.
See! What the heck is it?

Anyone? Anyone?