Was it grwoing in shallow water or maybe very wet ground? It looks like Azolla caroliniana. This plant grows a floating aquatic, forming dense mats, which could easily survive on a wet substrate. It is very Selaginella like in apperance, but much more compact. It does develope the blue/purple/red color similar to S. unicata under strong light. It is fairly light demanding and I have had little sucsess with it in the terrarium. The plant is widely used in water gardens (probably mostly by accident) so its found in lots of wierd places.
Then I will say I am almost 100% certain it is Azolla. It truely is a neat little plant. A close relative of the ferns. I will make suggestion though, if you want to use it and keep it on hand you should set up a small aquarium to culture it in. I do this with Salvinia minima, another "floating fern," and I always have a ready supply for use in water features or on a bog substrate. Good luck.