SD Internal make my Motorola X Play 3rd generation reboot and unmount sdcard I bought the fastest 32 GB SDCard : Sandisk Extreme USH-I card ( Ultra HD )

Then, I formatted it as Internal Storage ( I have Android 6)

Now, my phone reboots sometimes, and also my sdcard is unmounted frequently.

So, a lot off apps doesn't work, because they need data on the sdcard.

The only way I found to fix it, each time, is to extract phisically the SD, and insert it again... this works for like 2 hours....

Is there anything I can do about it???
It is not uncommon for this to occur if the write speed capabilities of the SD card are not sufficient, we have seen many examples of this with the Moto X Pure/Style and Moto G 2015 on XDA where a U1 card does not have fast enough sustained write speeds, and replacing it with a U3 rated card works beautifully. I am not personally familiar with your Moto X Play, but it is very similar to the G series hardware-wise. 

This could also be an few other issues, like a poor connection to the cards contacts. Sometimes placing a couple layers of scotch tape on the back of the card can increase tension enough to keep the card in contact with the tension contacts in the microSD slot.

This could also be a fake or counterfeit card, you can use SD Insight to check it.

Not to count out it could just be a defective microSD card, or defective hardware, but you would have to eliminate and try the other issues above before moving to this conclusion.