Some orchids have trouble with water if it sits on their leaves for too long. I've seen Phal. leaves rot because of water sitting on the crown too long. 'Crown' might not be the right word but I can't think of one that is better. Try to avoid leaving water droplets on the leaves. The end of the one leaf probably is infected but I expect it won't spread much more than it has as long as the plant is in good growing conditions. Unless the infection continues to spread on the leaf toward the crown of the plant then I wouldn't worry about it. Over time the spots might blacken. Phal. leaves don't last forever so eventually the leaf will die back naturally. If you're really worried about it then the simplest control method would be to trim off the infected part of the leaf or trim the entire leaf. I prefer the wait-and-see approach.