What is it and what do I do with it?

My guess is you could probably get it to grow on a rock if you wanted it too. It's nice looking stuff though and I wouldn't mind some of that for my tank!
 
Looks to be fissidens fontanus. Not sure if it can be attached to rocks, as I've never kept it myself.
 
It should attach to and grow on holey rock quite well.
 
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