Looks like sunburn, where are you in the world and where do you have the orchid. What sort of mixture do you have and how often are you watering the orchid.

It's always been in a kitchen window with other orchids (none of which were affected). I'm in the Midwestern US. It's potted in orchid medium (mulch like) and is probably watered 2 times a week or as needed.

I think minis are harder to care for than larger phals ; they need more frequent watering for sure. I'm wondering if it's been underwatered.

This looks like a fungal or bacterial thing.

Here's a link:

http://staugorchidsociety.org/culturepests-diseases.htm

Frankly I'd cut the whole leaf off and check the crown for the same thing.

I'd say that the bark mix is staying too dry. Or should I say that it dries out too fast. I had the same problem, no matter how much I watered them. Now I use a potting mix of 50% orchid bark mix (fir bark, charcoal, & perlite), and 50% sphagnum moss. Not just any sphagnum moss, either. Make sure the sphagnum moss is either of the Chilean or New Zealand varieties. Using this 50/50 mix takes full advantage of the best qualities of each medium; the open airy texture of the bark mix, and the water absorbing capabilities of the sphagnum moss. This lets it hold on to some moisture, while still letting the roots breathe. This blend is pretty foolproof. The bark mix prevents the sphagnum moss from becoming too compacted, no matter how firmly I pack the mix into the pot. And the moss prevents the bark mix from drying out too fast. Not to mention the fact that you can't "overwater" orchids that use this. This mix works so good, I now use it for all of my orchids.

I don't have any photos of them right now, but all of my mini-phals have been repotted with this mix, along with the rest of my orchids, and they've sat outside all year long. The only care they got from me was the occasional blast of water from the garden hose.