Diagnosis
You have accidentally enabled the Black Stabilizer or FPS Counter feature, which includes a dynamic on-screen crosshair, or you have enabled the dedicated Aim Point feature.
How to Fix It
Press the joystick on the monitor to open the OSD. Navigate to the Game Adjust menu. Scroll down to Aim Point. Toggle it to Off. The crosshair should immediately disappear. If it does not, return to the Game Adjust menu. Find Black Stabilizer. This feature dynamically adjusts gamma and can include a visual reticle in some modes; setting it to 50 (the default) will disable any auxiliary visuals. Also, check the FPS Counter setting in the same menu; while it shows numbers, ensure it is also set to Off.
If That Didn't Work
Perform a full reset of the monitor's settings. In the OSD, navigate to General. Scroll to the bottom and select Reset. Choose Yes to reset all settings to factory default. This will turn off all game features, including Aim Point, and reset your picture modes.
Heads Up
The Aim Point feature is stored per picture mode (Gamer 1, Gamer 2, FPS, etc.). If you switch picture modes, the Aim Point setting for that new mode may be independently on or off. Disable it in each mode you use if you find it re-enables itself.