Strange extra frequency in crystal oscillator
This really looks like a sampling artifact on your end, not something the crystal is doing.  Expand the scope time scale (lower time/division) until you only have a cycle or two per division at most.  If it's a sampling aliasing problem, then the artifact should disappear.

Alternatively, look at the signal with a Ye Olde Analog scope.

If it turns out it's a sampling artifact, go back and read up on sampling theory, paying particular attention to what Nyquist had to say.  Also learn about "aliasing".

Basically, a point sampled stream can only preserve frequencies up to half the sample rate.  Higher frequencies than half the sample rate look like lower frequencies in the input signal.  Put another way, after sampling everything looks like a frequency from 0 to half the sample rate, whether it was in that range before or not.

In practice, it's good to have a healthy margin between twice the highest frequency in the input signal and the sample rate.