Unfortunately, this is what happens when an extremely powerful magnet is left in one position on the cell for a long time. In this case is was only a few minutes, but a very strong field can cause the coating (surfactant) on the nanoparticles to 'burn'. Surface plasmons are, as the name suggests- a plasma and the 'nano heat' they make can cause this problem.
This action leaves a residue on the glass that will render that portion of the cell quite blurry and limit the distance the light bands can travel.
One solution is to heat the cell to about 160 deg F and place on a hard, flat surface. Then use a soft cloth to rub the affected area while applying lots of pressure. Rub around in small circles until you see the particles disperse. You may have to repeat this operation a couple of times.
If this technique doesn't work, then its time to box it up and ship the cell back to me for a re-build. Each cell has a 2 year warranty, including this type of defect. I've destroyed a few cells this way, too.