I think coralbandit might be on to something there. I'd take a razor blade over it to see if it'll come off. If it is a crack, I'm guessing unless it gets worse it probably won't be an issue. If it's a crack I'd take a sharpe and make a mark at the end of it so you can see if it is getting any longer.
This actually reminds me that I noticed I had a small crack in one of my tanks after I got it all setup, cycled and fish in it....

I made a mark, and it didn't get any bigger for months, then I forgot about it until... now. lol

I'll have to see if I can find it again, but about 4 years later I haven't had any issues.