what is this stuff?

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Based on the fact you took it off a filter at your local fish store and the looks of it from your picture, I would say 100 to 1 it is hair algae vs some kind of moss.

It's not impossible to get rid of once it is in your tank. If it starts growing on its own, that is due to you aquarium parameters & the tank environment, which is what makes it difficult to get rid of.
 
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