SD Internal make my Motorola X Play 3rd generation reboot and unmount sdcard
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.