does anyone know what granny called the "concrete pond"???

Nothing.

The mansion did have a swimming pool which Granny called the "cement pond".

I have a question that I always wondered
about, speaking of the cement pond....how
did Elly's seal, walrus, etc live in
clorinated water? I always wondered about
that....Also, did the thing have a diving
board? I never saw one....Jethro always
belly flopped in....

Awwww...c'mon now. Do you actually envision the Clampetts adding chlorine to the cement pond? Do you think they even ever heard of chlorine?

There was one show where a man who was supposed to put chlorine in the pool was chased away by Granny with her shotgun,as she said,"Quit tryin' to poison our pond!"