In a little more detail: the mouse pointer on your screen usually looks like a white arrow. The "Custom Cursor" is an ornate version of that mouse pointer. See the screenshot below for a look:
The "Custom Cursor" is shown inside and to the right of the green circled area.
Sometimes, this feature can cause performance problems. If you have any trouble with it, you can turn it off and switch back to using the white-arrow mouse pointer. You do this by clicking on your user name (or Guest #) in the upper right corner, which opens your profile settings. Just uncheck "Custom Cursor" to turn it off.
The related bug caused the "Custom Cursor" to turn back on unintentionally. So if you were having this problem, you shouldn't see it happen anymore.