Tough to say without seeing it. Remember, that what you see is on the outside. The important stuff is on the inside. Of course, rust anywhere is not a good thing. Rust never sleeps.
As it takes only a few moments and zero technical skill to clean and lube a chain properly there is no reason to let it rust. So my advise would be to clean it (some orange cleaner and a chain brush and some elbow grease work wonders) and give it a closer look. If it was light surface stuff you could tell that after getting it clean, which it sounds like it needs anyway. Then lube it if it is only 'a speck'.
I don't guess about the chain, I give it a visual after every ride and wipe/lube/wipe every 2 or 3 rides. Every 5 or so it gets a full cleaning and I use a chain-checker regularly. Call me neurotic about it but my chain looks as clean as the rest of the bike. About a month ago, the chain on my MTB actually had a bad link- it was literally pried open at one link and about to come apart. It might have taken out my new XT rear derailleur had I not checked the chain and just ridden it again. Maybe I am not so neurotic after all.
Tzvia- rollin' slow...
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