Most definately not a mushroom. Looks like it could be an anemone. In which case you'll need to return it asap. Also, you might want to consider a different lfs, he either doesn't know what he's talking about or doesn't care and just wants to make money off you. Or, if there's no other lfs's around, do some research on what you want before going there so you know what you're looking for. (Although with a 5gal tank your options are kinda limited)