It’s a fungus as many have said. You can kill it with UV light, but it will still be inside the lens. If you’re handy and are willing to disassemble the lens, then you can submerge the lens piece in hydrogen peroxide and then gently clean it off. 2nd option would be to send it off to a repair shop. There is also a chance that even if it’s cleaned off, that it may have etched itself into the glass.