I don't think its a weed growing out of a fuchsia, but I do think the fuchsia has a problem which is causing strange growth in certain parts. Looks almost like the flowering shoots are badly affected, so I'm wondering whether its fuchsia gall... Picture's not fully clear, its a dark background, but it does seem that there are little brown parts on the ends of the strange shoots.

Steven143 post a photo of an Angel Wing Cane Begonia yesterday and it struck me how similar to yours it was?

It looks to me like a weedy member of the mustard family, but I haven't tracked it down, yet. There can't be that many members that have opposite leaves in the juvenile stage.

Hmm, interesting - its hard to tell for sure from the picture because its dark, but the plant is strongly reminiscent of one of the willowherbs - these pop up in my garden and are weeded out relentlessly, but its not impossible that the original fuchsia has died, the central core has sufficient loose stuff in it, and a seed has blown into it and grown. I've certainly seen broad leaved willow herb growing out of a very thin pipe in the ground which, as far as I could see, didn't have any soil in it...