Diagnosis
Toner spillage or dust is normal over time, but a large amount usually means a cartridge was mishandled (shaken side-to-side) or a seal was broken. It can cause background spots on prints.
How to Fix It
Unplug the printer. Open the front cover and remove the toner cartridge and drum unit. Use a dry, lint-free cloth or a vacuum with a hose attachment to gently clean out the cavity. Wipe the metal guide rails inside. Do not use compressed air—it'll just blow the fine toner deeper into the machine. Clean the corona wire inside the drum unit (the thin wire you can access by sliding a small blue tab back and forth on the drum). Reinstall everything.
If That Didn't Work
If you're still getting random black spots or smears on prints after cleaning, the fuser unit might be contaminated with melted toner. That's a hot, more complex component to clean or replace.
Heads Up
Toner is not toxic, but it's a fine plastic powder. Don't create dust. Clean up spills with a damp cloth, as water neutralizes the static charge that makes it airborne. Never use a household vacuum without a filter—toner particles can pass through and get into the motor.