How do I unmount this Phal. violacea and move it into a pot?
 

I have this Phal. violacea that has overgrown it's small fern board, and now I want to move it into a pot. How do I do it?

And also, it has these kind of dark spots under it's leaves, I wonder what it is? (And what caused it?)

You cannot remove it from the plaque, as the roots are all through it.

It has not really outgrown that; it'll be fine for years if you're capable of giving it the care it needs.

If you MUST pot it up, only trim away the parts of the tree fern contiining no roots and pot it up with the root mass and tree fern as-is.

I doubt you'll get a decent diagnosis with a better description and photos of the issue.

The dark spots look like spider mite damage.

I would just set the mount into a pot horizontally with a little media underneath. For me mounting is absolutely not an option so I have potted several mounted plants. I just leave the mount intact or break off parts that are not rooted and pot the whole thing with the plant facing up

All the previous advice sounds like the best that can be done. As trying to remove any of the roots will just be a bad experience!

I agree, just put the whole thing into a pot, roots will eventually go out into the pot.

just pot the mount

I'll echo that this one has definitely not outgrown its mount and that if you want it in a pot, dropping the whole thing into a pot and adding media is your best bet. However, the problem with dropping it into a pot, is that you'd probably put the long part of the mount into the pot, thus "planting" it upside down.

It looks like it is growing so well on that mount... if it has grown like that in your care, you have the conditions to maintain it that way. Why change it? I agree that it has certainly not outgrown the mount. And if some roots do go out into the air, they are doing what they do naturally. Where you live, I think you have both heat and humidity, which it loves. So why mess with a good thing?