The 'weed' looks like a violet of some species and does, indeed, seem to be pretty lively. It is likely that it has lots of shallow roots spreading around so you must ensure that you get all the broken bits of root out.
The acer will need lifting with plenty of soil around the roots and it may be easier to weed when this is sitting on top of the ground. Do you have to lift it now as this is not the best time of year as we move into the hotter and dryer season (yes, I am laughing as I write this). Moving it in late autumn would be better if you can.

Thanks bulbaholic, i must admit i havent looked at it properly yet and sorry forgot to mention it's in a pot.

then it will be a bit easier to rid the weeds. Lift the plant out of the pot and carefully scrape the top of the compost off. plant and then add soil to the original soil level on the trunk.

Thanks Sbg :-)