If it is riccia, you will be able to easily remove the netting holding it onto the rock and split the riccia portion in half. Then attach a thin layer of it to both of the rocks that you want it on, using the netting or some fishing line. If it is a moss, such as java moss, you can use the same method, but it will grip onto the rock and doesn't need to be permanently fastened. Riccia doesn't "attach" to anything and will float away if released.
It is hard to imagine you would confuse a moss with riccia though, since they really don't even resemble each other.
Two 1 GA desktops, a 5 gallon, a 2.5 gallon, a 10 gallon, a 20 gallon, a 25 gallon, and two 75 gallon tanks, all planted.
I am an expert on algae, so ask me if you have questions. I know how to grow it, just not how to get rid of it.