That which you are seeing is a light diffraction 
pattern or light scattering that occurs when two 
pieces of transparant material come extremely 
close together.  This can also be observed with 
other types of plastic such as acrylic.  I do not 
know if this is damaging, but I would think it 
would not be hazardous to the cel.  It just 
indicates that the cel layers are very close 
together.

Craig