Unless the slime the researchers planted was from the slime-mold being experimented upon we can assume that they react in the same way to other slime-mold's slime as to their own.
Can they really be said to be 'memories' then?
I doubt that most people would consider Hansel and Gretel's trail of breadcrumbs a form of 'external memory.'
Could you elaborate on what an 'external memory' would be then?