How do I get rid of this brown stain?

[KCD]

There's a line of brown stains that I've been trying to remove. It looks like rust (see pic above). I tried muriatic acid, chlorine puck, and vitamin c tablet without success. Any ideas? I'm thinking it might be rusted steel (rebar) coming though plaster.

 

[Texas Splash]

Try holding a strong magnet against the stained area to see if it tries to grab rebar underneath.

 

[KCD]

Good news! A strong magnet did not stick so rebar is not exposed. Bad news is I still don't know how to remove the stain.