It might be fungus, but a similar-looking haze or smear can also be residue from plasticizer outgassing rather than fungal growth. Either way, lens fungus grows slowly and is unlikely to directly “infect” your camera body or other lenses in normal use.

Start by checking whether the mark is on an outer glass surface. If it is, try cleaning the exterior surfaces carefully. If the haze or growth is internal, repair usually means professional disassembly and cleaning. On many older or lower-value lenses, that can cost more than replacing the lens.

So the practical approach is:

• clean the outside first
• if the problem is inside the lens, compare cleaning cost versus replacement
• don’t panic about your Nikon F3 or a replacement lens being immediately contaminated

If the lens still works acceptably for your photos, you may choose to keep using it until replacement makes more sense.