You can try to clean the stain but it's almost certainly set into the clear coat.
Unless you are going for a full rebuild, you may just want to degrease it more and live with it. If not, you will need to strip it off and deal with the raw aluminum, which will require you to paint it, powdercoat it or re-clear it -- depending on how far you want to go. Some guys pull the motor and spray it with VHT paint, as an easier fix. Don't leave it polished and bare, as you will still need to periodically polish it.
I would say either degrease it more and live with it if you can.