Simple Green is a highly concentrated cleaning fluid. When correctly diluted it can clean pretty much anything, including cleaning the engine, cleaning wheels.
Simple Green can corrode the aluminum in your engine bay, but only if you use the wrong concentrate over a period of time:
"Thank you for your inquiry. Under certain circumstances Simple Green can corrode aluminum - but it doesn't automatically do so. Corrosion typically occurs when the product is used in too strong a concentration and/or is left on the surface for too long."