I don't think that's a crack in the aluminium, but if you really want to be sure you need to stress the frame and see if the crack moves. The best way I can think of to do this would be to clamp a small camcorder to the frame, set it recording, and ride the bike for a bit (on an upward slope, in a high gear).

If you don't have a suitable camcorder, many cameras or cellphones will record video with adequate quality. Make sure the defect fills the frame so that you can see any flexing going on. Fixing the camera will depend on what you have. I've used miniature G-clamps before: attach the clamps firmly to the frame, and attach the camera to the clamps with large elastic bands.

If the bike passes this test, I wouldn't worry any further. Just check from time to time that the defect isn't growing.