Josh,
It seems something like logo/watermark from the server. Just by simply consuming the WMS from a browser and we can see this grey rectangle, please try the link below.
hsservices.datadoors.net/__s...HEIGHT=256
The WMSOverlay / WMSRasterLayer are getting the images by tiles so every tile is an independent query and it has that grey rectangle logo things. If we switch to SingleTile it will only have one on the bottom right but it might not be very responsive for loading a big image one time.
wmsOverlay.TileType = TileType.SingleTile;
I am not sure if the website provides something like premier users without any grey rectangles, that will be the best solution I think.
Thanks,
Ben