my iphone x some times it shows an x on the battery icon.why?

I saw that on a friend's iPhone X before and I think we realized it was caused by an app he was using that had a screen overlay. The x was to close the overlay and I believe just happened to coincidence with the battery's position.

I've never seen that before, and there is no indication or explanation for that in the User Guide. Do you have the device plugged in for charging? In that case, you should see a small lightning bolt in the battery icon, but I've never seen an X. Can you provide a screenshot of this for identification?

Sounds good. Will help in trying to troubleshoot the source.