This is normal, it is usually an indicator that you are nearing the end of the cycle period(diatom algae) Its not actually algae but rather the skeletons of diatoms all linked up. It is a good thing because it precludes the start of the good "green" algae types. When it is gone, you are complete. You can keep cleaning the glass though, try not to disturb the subsrate though, let it run its course:P
Hope this helps
Don't tap on the glass, the fish will think you are stupid!!