I got a gear cassette that got a nice amount of rust on it.
Is there a way to clean it up?

If you hit it with WD40 (I'm going to get killed here) and wipe it down, I suspect it's mostly cosmetic.

WD40 and a wire brush

If it is much more than superficial, you can probably replace it instead (look to the various on line bike shops for cassettes (or freewheels).

Not from me. But I'd add in the same wire brush as suggested by Randallissimo.

I have used muratic acid several times, rinse off really well with water, then light coat of oil. It eats the rust and leaves the metal

Only do that on steel. It might attack alloy parts.

Bleach will remove rust. Buy liquid pool chlorine -- it is stronger and the same chemical. I have used it to remove rust from painted surfaces. Don't know how it would work on bare steel.

Boiled linseed oil could keep the rust off.