M_alu_result[10] was determined as clock . . . Why?!?

You don't want to put your 'report_timing' commands in your SDC file. You can either run them from the TQ GUI (from the Tasks window), or type them in the Console window at the prompt, or better yet, put them in a Tcl script and run the script either from the menus or at in the console window with the command 'source <filename>.tcl'. If you can find the source of this clock with report_timing command (or even report_path commands), then you may be able to determine its actual frequency. You assigned this clock the frequency of 500 MHz by declaring it to have a period of 20 ns with the -period option on create_clock.