|Thank you, PO|
This is (or was...) a brass fitting. It has de-zincified.
Brass is an alloy of copper and zinc (not to be confused with bronze, an alloy of copper and tin). Now, we put zincs on our props and/or shafts to protect the bronze props, right? Well, with brass, there is zinc right there, internally! And just like with your prop, in salt water the zinc dissolves to protect the copper. But when it is a solid piece of metal what does this mean?
It means just exactly what you'd imagine... This piece of used-to-be-brass is now copper, but with 30% void space - it is atomic lace. Of course it has no strength.
Brass has no place in boat plumbing (except perhaps in the fresh water lines at the galley sink). But sadly, most marine chandeliers not only stock brass fittings, but they also locate their brass and bronze fittings right next to each other, or worse, intermixed with each other. And since brass and bronze are similar in appearance, this arrangement infers that they are interchangeable. NOT SO! In fact, I will make the case that marine chandeliers should not stock brass plumbing fittings at all. Just like with non-stranded electrical wire, if a boater needs a brass fitting, he should be forced to go to a conventional hardware store so that it becomes explicitly obvious that he is getting something not suitable for marine service.