Few questions (again LOL). Does it feel soft/mushy? If you hold it against the light, what does it look like? Are these patches getting bigger?
If it feels soft/mushy it prolly is a bacterial infection that could spread to other plants (for instance during watering and leaf contact). You can watch it but to be sure you could remove part of that leaf. Use with desinfected tools and don't forget to desinfect afterwards as you don't want to infect the next plant you use this tool on.
I agree with Nicole. It may have got an infection, in which case I would cut it off.
Make sure you use sterile tools and gently dab the cut end with cinnamon. Not only does the cinnamon help it dry (don't use much on roots as it can dry them too much) but it also has an anti-bacterial effect.