Seems like dead pixels or stuck I don't really know, but remains even when watch is turned off. I have bought only 3 months ago for normal use, nothing extreme. My old one didn't have any problem for 3 years. For 300 euros not good.
Other users have already replied this, but we would like you to reach out to your local product support channel via support.garmin.com so we can create an exchange for you. We are unable to set this up through the forums.
We apologize for this inconvenience but we will get a replacement out to you as soon as possible.