White spots/blister on Phal leave

[franfrancy]

Hello everybody!
I'm a beginner on orchid world and I need your help.
Days ago some spots appear on the leaves of my Phal. I use MSU fertilizer, weak solution, I water the plant around once a week, I put the orchid on the sink, flush with water and then apply the fertilizer. It's potted on a clear pot with bark mix(I got it from repotme).
Also the plant is located close to the north window, in a very bright room, but not on direct sun.
So far the plant seems doing good, new stem is growing and the roots are very healthy, but the spot worries me, I had searched around and I could identify what is the problem.
I'm sending some pics and thanks a lot for any help!
Ah, I forgot to mention. I live in Chicago.