Diagnosis
That's fuser ghosting. A speck of toner or debris is stuck on the hot fuser roller, re-melting onto each page at the same spot as the roller turns.
How to Fix It
The fix is to replace the fuser unit (FM-350). Turn off and unplug the printer, let it cool. Open the rear cover, lift the green handle, and slide the fuser assembly out. Replace it with a new one.
If That Didn't Work
If the ghost image is a faint copy of the previous page's content (not a blotch), that's 'toner ghosting' and points to a worn drum. Replace the drum unit for the color that's ghosting (DR-330 series).
Heads Up
Don't try to clean the fuser rollers yourself. They're coated with a non-stick layer you can easily scratch, making the problem worse. The fuser is a wear item meant to be replaced.