When you move the projector left-right, does the artifact move with the projector? If yes, you left something behind in the projector... a water spot or blob of dust probably. If the artifact is on the top of the screen, it will be at the bottom of the lens or other optical path component. If the artifact does NOT move with the projector, something has rubbed the paint you are using for projection causing a loss of reflectance in that area. Repainting that area MAY solve the issue, but it COULD leave a visible dividing line around the edges of the repainted part. So if repainting and feathering the edges of the repainted area leaves anything visible in the screen paint, you may have to repaint the entire screen area.