Bright green bar appeared on top left of my iPhone X

First try a Forced restart

https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

If no better

You should arrange a repair

https://support.apple.com/iphone/repair/service

or: contact Apple Support 

Anything other than Screen or Battery may be a replacement phone and the price is in the article.

You can see additional service information within the following article:

Prepare your iPhone for packing and shipping

A screenshot may help because there are a number of valid green icons caused by personal hotspot or live phone call etc

https://support.apple.com/en-gb/HT207354