to get rid of it you're probably going to have to shave/sand it off. Since the aluminum has bonded and oxidized, the components are now stuck together. with some sanding, you'll be able to remove the "crusty" layer, and then you'll have to protect it with a kind of metal polish (as posted before)
some of the parts on my 996 looked like that. I used windex with 000 steel wool vedry carefully. this will take off the oxidatin but it has to be done gently it will scratch the aluminum. Get your hands on purple metal polish. I havent used anything better on aluminum. Its about $20 for a 6ox bottle, worth its weight in gold.
I have a 1982 Yamaha XS400rj with the polished aluminum engine cases, when i got it the whole engine looked like that, I bought a 7 dollar tube of AUTOSOL and it has worked miracles. It does take a TON of work but i figure if you had like something to go on the end of a electric drill it would speed the process up. Its a great product and also protects the metal greatly afterwards