The single line of text is most likely print commands. This sort of thing can happen when you use a printer language/driver that the printer does support. An example is using a Postscript driver to a PCL printer. Checking the tech specs for the 6P, the printer does only support PCL while the 6MP added support for Postscript.
So what you can try is opening Print & Scan, selecting the LJ6P in the Printers list and removing it by clicking the minus button under the Printers list. Then click the plus button to add the printer again, select it in the list but then you need to change the Print Using menu from the 6P to Generic Laser Printer. Click Add to complete the queue creation and then see if this will print from the iMac.