As the title said, the ghost on water doesn't mark item on the floor to be deconstruct, leaving ghost non buildable sometimes. (you have to manually deconstruct the said item on the floor). I discover that because my artillery let a lot of stone on the floor (destruct rock).

I'm sure if fixing this is worth it, especially since fixing it would have a not insignificant impact on performance (I'd have to check for items on the ground every update). It's also quite easy to do manually as its just one drag over with a deconstruction planer. If you really want to automate it nanobots has a way to tell your bots to deconstruct all items on the ground.

Do you mean items dropped on the ground, or cliffs and trees?

Items dropped on the ground can be marked by a deconstruction planner. The symbol is a white box above a line, in the tab that's a box with question marks. (Way, way down the list, below all the ghost entities. Search for "item on ground".

Yeah, I know the workaround. It's fine to not fix it if it's too much performance. The cliff/rock/tree are already marked to deconstruction so no problem on this side.