The most likely cause is stray light entering through the DSLR viewfinder rather than through the lens. On a Nikon D800, close the built-in viewfinder shutter or cover the eyepiece; that often eliminates the purple haze. This kind of leak can show up as a magenta/purple cast, especially in very high-density filter situations.

A second possibility is the filter itself. Some strong ND filters can introduce an uneven magenta color cast, particularly lower-cost ones.

To troubleshoot:

• close or cover the viewfinder and retest
• check for any other light leaks around the camera
• compare with and without the filter under the same setup
• if the haze remains even with the viewfinder blocked, the filter may be causing the color cast

For eclipse/solar work, always make sure the filter is designed specifically for safe solar photography and is used correctly.