Prepare a solution of water+5% (in volume) alcohol.
Soak lightly a piece of white cotton and rub the underside of the leaves.
If the cotton gets redish/brownish then it's a case of spider mites.
The scar on one of the leaves is mechanical damage and it doesn't cause any worries (but it will remain like that).
One of my Phals got a bit too much sun exposure and developed some spots like this. Maybe evaluate where it’s sitting? Mine was fine during the winter but with the shift in light it started getting some direct sun in the mornings while I was at work and it took me awhile to figure out what happened.