When you say "cursor", you mean the mouse pointer, right? If so, this is indeed a bug in the fglrx driver. For me, this bug only appears when I don't have DRI (direct rendering) properly configured. With DRI correctly set up, I don't see this junk below the mouse pointer.
See also http://ati.cchtml.com/show_bug.cgi?id=252 -- When I log in right after the system boots, there's a weird screen dirt underneath the cursor https://launchpad.net/bugs/46512 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
