GIBBANATOR From what I can see in your pic, the green patch sort of looks like an artifact of a deleted pool that had the stagnant texture (like the pool on the Windenburg Bluffs lot). Based on what you said about moving the house to a new lot though, it also seems to be attached somehow to the house since it stayed with the house when you moved it to a new lot. So, my 1st suggestion is to place a household on the lot then move them and keep the furniture. If it works and gets rid of the green patch, you'll have to refurnish the house though. My 2nd suggestion is to place a pool in the area where the green patch is and see if that does anything. If it is a leftover pool water texture from a deleted pool, it might become accessible, and you can delete/change it, and it stops showing up after you delete the pool again. Downside of this is having to tear down and rebuild part of the house.
Hope this helps.