It's algae, so cutting it in half won't harm it any. They're formed when a piece of an algae mat breaks off and is tumbled around by waves. So it's good that they're playing with it and constantly rolling it around. Cutting them too small might make them more of a "snack" than a "toy", though.
It's important to clean them once a month or so, too. Just squeeze them out like a filter sponge. Otherwise they get crud built up in them and other forms of algae start to grow on it which kills it off.
It's important to clean them once a month or so, too. Just squeeze them out like a filter sponge. Otherwise they get crud built up in them and other forms of algae start to grow on it which kills it off.