That is an extremely cool cel.  I wouldn't let a 
little tape bother me at all.  If you want to bag 
the cel, then Drac had the good idea to just 
slice along the edge of the tape connected to the 
cel.  Most cel acetates extend beyond the cel 
image/image meant to display anyway.

Try collecting cels from an old series like 
Patarilo, where the original owners backed the 
cels with some kind of tissue/tracing paper!  And 
not full-page size sheets, either!