They never really mentioned it specifically; to me they kind of look like coral. I loved this episode of Pokémon; they put effort into making a special mysterious side story. I hope we will get more episodes in the future that teach us about Pokémon in ancient times. These are things we generally don't see in the games; I find it very interesting, however.
Me too, haha. Then eventually Superpuff came to the rescue! Also, didn't they get eaten by Gengar in this episode? I think I remember that the group and their Pokémon were crammed up with Team Rocket inside Gengar. I've read about Pokémon eating other Pokémon before, but people? xD