If you mean the dark green stuff in the centre, it appears to be a nice little clump of one of the liverworts.. it's a mossy type thing grown the same way, most will cling to rocks or wood, can be tied on. Fragile leaves break easily but will all grow new leaves.
There is a small one, a mid size one and a giant type. The littlest one is often called mini pellia, the mid size just pellia. One of the 'giant' types is called Subwassertang. Main difference being the width of the leaves.