Welcome to Android Central forums.
Are you able to push it, like a button, what happens if you do?
Are you able to take a screenshot of this so that we can see it?
What are you doing when it appears?
Is it always there, even when you're not touching the screen?
Initial thoughts are that this is the PIP close button. It sounds like something you would see if you were watching YouTube or had some other app open in a picture in a picture window. This would be where you'd drag and drop that window to close it or remove it from your screen.
Without a screenshot and answers to our questions, we're left to guess. Help us help you.