It most likely looks like fungus. In cases like this, cleaning may remove some of it, but fungus can also damage lens coatings or even etch the glass, so the mark may not fully disappear.

Because the spot is near the center of the rear element, it has a better chance of affecting image quality than a small defect near the edge. Possible effects are reduced contrast and some loss of sharpness, which may be more noticeable at smaller apertures.

The practical test is to shoot a roll and compare images made at different apertures and in different lighting conditions. That’s the best way to see whether the lens is still acceptable for your use.