Probably not rust!
This sounds like the problem we all have with the budget end of saxes. Flux bleed.
This is where the soldering flux hasn't been cleaned off properly before lacquering.
Nothing much can be done just live with it. Others may have different takes on it though.
Could be, as Andy says, from soldering, but if it's away from solder joints, like inside the bell, and is red, it's the zinc leeching out from the brass by electrolytic action, leaving behind a red copper oxide. Clean it off with a mild metal polish and wax/relaquer as appropriate. If it's green, it's verdigris, clean off with vinegar and then polish.
As for quality, if the Stagg clarinet we bought our son a few years back is anything to go by, it's very basic entry level kit. Price sounds OK, though.
My 1st sax was a Stagg 77 alto and it done me for 21/2 yrs and at the price you paid if it plays well then you have a bargain imho, they are as Kev says a basic entry model sax but they do play well (my alto does)