While I can't say for sure, I'm going to venture a guess this is the same stuff that slowly builds up inside surfaces of the SS inflators on my Halcyon wings (yes, I know they'll eventually try to kill me; no, I don't care because I like them too much). Basically, it's rust of ferrous scale building up on the SS component, deposited by the water in which you're diving--the SS component itself is not rusting. Given that this is a part where that stuff is being ground between the threads, I'm not surprised it's hard to get off.