You need to remove all the weeds/garbage around it in your town, and most of the weeds in the rest of your town
will take a few days to die, and then will go
From the pictures, I thought it looked like a vileploom, but eh, close enough.
How long have you been neglecting your game? I once had a TT accident and traveled 29 days, flooding the town with weeds. I still didn't get that thing. I did have a few flowers still leftover so maybe that's it?