This white spotting is usually caused by dehydration and is not an effect of hydrogen peroxide. It may be more likely to occur in some individuals based on their tooth genetics, diet, or tooth health.
It is important to note that this spotting is temporary and not harmful. It is okay to continue with the course of whitening strips and the spotting will even out naturally following the treatment course, If you have any concerns you can check also with your dental professional.
All of the best tips and directions are included in the whitening strips user manual which you can find here.