I'm no expert, but the green looks like algae and the darker places look like fungus, but the fungus looks like it is still manageable at this point. I would treat with whatever fungicide you are comfortable with, and try to avoid water standing on the leaves. Good luck!
I haven't used fungicides heavily, but soon I will be taking it up a notch for the rainy season that is upon us here in South Florida. Right now I am soaking everything once a week in 1/4 strength diluted generic Listerine. It contains thymol, which is suppposed to be a systemic fungicide. You can also use pool algicide, called Physan. I haven't used it yet, but that will soon change.
It might just be an older leaf that's dying off. That sometimes happens and it happens on some kinds of orchids more than others. I don't grow bulbos so I don't know for sure. The one new leaf looks very healthy. It's possible that there's nothing wrong with the plant. If you want to start by cleaning off those leaves just use soap and water. Safer's insecticide soap is available at Home Depot or Lowes. You can clean the leaves with Safer's and kill any mites that are on the plant at the same time.
I suggest you do it with :
- sodium bicarbonate 1 teaspoon
- soap or vegetal oil 4 teaspoon
- water 4 liters
Shake before use with oil.
It's fungicide (mcGill university formula) and won't arm the leaf. The solution won't last months so prepare only what's needed. I use the oil, not the soap for this, as in these conditions the soap makes way too much bubbles and things like sphagnum keep wet a long time…