Does it react if you touch it? Does anything happen if you touch it, the vol goes up or down, can it be moved around on the screen, what happens when you touch it?
Not knowing your phone brand, could it be an indication that an Accessibility feature is turned on? Check your Accessibility settings under hearing impairment options for anything turned on. You might also check the visual impairment options as well for anything being turned on just be sure it isn't something else.
Let us know what you find.