I would give it a good healthy dose of CLR ( it's in a bright green bottle, Calcium Lime & Rust), and set it soak for a while 30-45mins then spray as much of it off as you can with the jet nossle on the hose handle.
That should take off most of it. Do it outside, cause it is super strong smellin'
If that dosen't work, maybe try a stemy hot bucket with landry soap and a scrub brush and hose off. Or maybe make a Borax paste, apply the paste, scrub and rinse.