That symbol is for your Command key, and tells you that it is set, and is completely normal - when you have Sticky Keys enabled.
If you press your Command key several times, you will see that symbol go through two different brightnesses, then turn off. It changes each time you press the Command key.
If you don't need that icon to appear on your screen, then turn off Sticky Keys.
Check in your System Preferences, then the Accessibility pane.
Then, in the Keyboard settings, uncheck "Enable Sticky Keys."
Be sure to check in the Options for Sticky Keys. If you have checked to "press the shift key 5 times to toggle Sticky Keys", then you can still inadvertently turn Sticky Keys back on.