I can give you two options, one super simple and easy, the other only slightly more difficult.

You could use a text character, perhaps something as simple as a period or an O, or something fancy from WingDings. Pick a size suitable for marking the player. Use the placement tab in the text generator and keyframes to move it around and follow the player.

Alternatively, double the video so that it appears on two tracks, one lined up directly over top of the other. Lighten the top track slightly and darken the lower track. Apply the cookie cutter effect to the upper track and use 'cut away all but section'. Reduce the size to match the player and choose the shape you want. This will highlight the player and darken everything else. Use keyframes to move the x & y coordinates around to follow the player.