I believe the ones that have the golf ball in the middle are MOSS balls...Petco sells some that are made of some type of moss such as java moss I think. They usually float around the tank for awhile. marimo moss balls are actually a type of algae.
@Tsoto: The fake ones will constantly float and if it is an actual ball or golf ball inside, you won't be able to squish them. Real marimo only float if you squeeze the water out of them, but will sink back again once water logged.
People asking questions about them in the lounge now? I give up, lol.