Black square icon always present on new 17

Since we can't see what you see, it's very hard tell what it may be or how to get rid of it.

Perhaps you'd like to post a screen shot? Or tell us a bit more about it? Like is always visible or in a specific app only? Does it do anything when you touch it?

Help us help you by providing some details about the black square.

That one looks like you need to turn off Hover Text at Settings > Accessibility > Hover Text.

Maybe Assistive Touch? You can check if you need to turn that off at Settings > Accessibility > Touch > Assistive Touch.

Otherwise a screenshot would be helpful.

That is Hover Text. As mentioned it can be turned off in Accessibility.

Hello! You can try to force restart the phone as that usually fixes majority of the issue!

Force restart iPhone - Apple Support

You are welcome!