Reef Safe Metals?

Titanium would be about your only choice.
Mini fridge compressors are not continous duty so do not stand up to constant use. Look at how long it takes to cool them down after adding a bunch of warm pop or beer, circulating warm water through one continously just does not work well. Its been tried a jillion times.

 

Every stainless steel contains a high proportion of iron (typically 70+%). It's the amount of the other elements that determines if it will rust or not.

If you use stainless steel, you need something with a high chromium content, like 16+%, and a high (12+%) nickel content. horkn is correct about wanting a 300-type steel, but you need to find out the composition of said steel, as the lower chromium varieties will rust when exposed to salt water, even if they're corrosion-resistant in air.