Light is reflecting off the back of the filter. You can plainly see the reflection of the front of the lens bouncing off the back of the filter.

Some type of makeshift hood may solve the issue if the light is entering in front of and passing through the filter. But I suspect the offending light is not coming through the filter.

With 100mm square filter holders, sometimes light can leak in at the sides of the holder through the small gaps between the filter and the lens in the areas where the "springs" that hold the filters in place are cut out. If the hood alone does not solve the problem, try covering the side holes with some tape that leaves no residue such as gaffer tape. If the top and bottom of your particular holder leaves gaps between the filter and the 'back wall' of the filter holder, cover those as well.

This review of the Sensei 100mm filter holder discusses what causes light leaks and ways different manufacturers and users deal with them. This review of the Haida 150mm system has some closeups that show the foam gaskets some makers put on either the edges of the filter or the edges of the holder to help prevent light leaks.

You can even buy self adhesive 100mm foam gaskets to apply to bare filters yourself. Be warned, though, they may cause (additional) vignetting with very wide angle lenses as they do slightly crop the edges of the filter to which they are attached.

Although it is not the cause of your reflection, Lee also recommends the viewfinder be covered when using their filters for long exposures. Light can leak around the raised mirror and get into the light box of many cameras, particularly if the camera is well used and the seals around the mirror assembly are worn.

This article by Paul Reiffer delves into the issue of light leaks with filter holders and shows example photos. It also include a comparison of three 150mm systems for his wide angle lens with built-in hood. The winner may surprise you.