Warren Moon -- RG3 Needs To Shut Up ... Stop Blaming Everyone Else!

NFL legend Warren Moon has some advice for Robert Griffin III -- STOP PASSING THE BUCK!

TMZ Sports spoke with the NFL Hall of Famer who admits he feels bad that RG3's "fairy tale" career has hit the skids ... but says the QB has no one to blame but himself.

"He hasn't helped his situation. All that social media stuff. You just can't do that. You have to focus."

The "social media stuff" Moon's talking about is this week's Instagram debacle -- when Griffin appeared to "like" an anti-Redskins post ... only to later blame the whole thing on an intern.

Moon says the situation is just another example of RG3 refusing to hold himself accountable when things go bad.

"He needs to take more blame and stop passing the buck ... even if it is someone else's fault. He's not popular with his teammates because of the way he handles things."

Moon adds, "He's the quarterback. And part of that is accepting blame when it's not your fault because you also get all the credit when sometimes you shouldn't."

Warren says he's tried to reach out to Griffin on multiple occasions -- but hasn't been able to get through to the guy. Still, he's offering one last piece of advice: