Looks like sunburn. If you think this is the case, move it out of direct sunlight ASAP. Phalaenopsis orchids only need dappled sun, and really can't take direct sunlight, especially mid day sun.
Looks like it may have started out as sunburn and then progressed to some sort of bacterial rot. I would cut that leaf way back into the healthy green tissue to stop the spread of infection to the rest of the plant.