Humans like to live in an oxidizing environment. Oxygen oxidizes a lot of things and released energy when it does. Humans and other aerobic life forms capitalize on this to get energy.
You want a life form that can oxidize anything oxidizable.
We have life forms on earth that can oxidize many things, including iron and just about every component of a living organism. Organisms can oxidize petrochemicals. I cannot think of anything that oxidizes metallic aluminum probably because it does not occur naturally but one could imagine a metabolism like that. Oxidation is a good platform for energy metabolism.
Your organisms are alien microbes that oxidize anything not already maximally oxidized: any metal with an oxide, any carbon except CO2. They are biologically unusual enough that they do not provoke a very robust immune response from humans. They are slow but they are tenacious. In an anaerobic environment they go dormant and wait.
For humans, slow oxidation would be a bad way to go.