Strange white line on equestrian center

@simprincesss  That kind of looks and sounds like a misplaced reflection.  Are there any items on the lot that could be generating that reflection?  Is it there only at night, or at any time of day?

If you use mods, specifically NRaas MasterController, does it help to reset the lot?  Click on the ground, select NRaas > MC > Reset lot, then go to a different lot, let the game clock run for a few minutes, and go back.  There also might be some leftover otherwise invisible item, although that's a bit more complicated to find.

If you don't use mods or don't want to bother looking for some invisible item, you could bulldoze the lot and replace it with a copy.  If you've made edits to the lot you want to keep, it's easy to save your version to the in-game bin beforehand and replace it afterwards.