Yep, what katschamne said. I too used to work for Petco and we got some of these so-called moss balls in without any info to accompany them. All of us in the aquatics department were puzzled as to what they were, and come to find out they're just java moss wrapped around styrofoam balls tied to a string. They will fall apart over time and are a complete nuisance, in my opinion, as the strings get tangled on everything. I had previously ordered a marimo ball online that looks nothing like what we had at Petco, and it sinks as it's supposed to. Plus, the real marimo ball has stayed in tact and looks great.
Personally, I recommended to customers to buy bunches of java moss and make their own "moss balls" since it was more cost-effective than purchasing our ready-made ones. Java moss is, what, $3.99 a bunch? The pre-made balls sold for $5 and up I believe.
I have always wondered how they attached the moss to the styrofoam though. It didn't appear to be tied on with string, but then again, I never cared to look that closely.