Even if it IS rust, TWD crew knows that we, the fans, will infer that it's blood. After all, this is a show about the undead with lots of gore.
Why else would AMC give us the rare opportunity to get up close and personal with the set?
Someone is being paid money to set-dress these rooms for us. I do think some of the pieces are just junk, but some pieces are definitely references and others may be foreshadowing future episodes.
By the way, if the saw is rusty, wouldn't the rust be around the screws attaching the handle to the blade?
We'll see if it comes into play. I'm just saying I'd have a hard time believing that if he used it for something sinister, that he'd leave a bloody saw there for all to see.