It may be fungus or mold, though one answer notes it doesn’t look exactly like the classic “spidery” fungal pattern. Either way, the lens has internal contamination and likely needs cleaning.

If your photos currently look good, the lens may still be usable for now. The main concern is whether the growth spreads or eventually affects contrast and image quality.

First determine whether the marks are on an accessible outer glass surface or inside the lens. If it’s only on an outer surface, cleaning is much simpler. If it’s internal, disassembly may be required.

Be careful: cleaning or opening a lens can easily damage it, especially if you’re not comfortable with delicate repair work. If you’re inexperienced, internal cleaning is best left to a professional.

So the practical answer is: yes, it could be fungus/mold, the lens may still work acceptably, but it should be cleaned if possible—especially if the contamination is internal or seems to be increasing.