Sargassum is a genus of brown macroalgae, with a fairly varied collection of species. As OrionGirl mentioned, "sp" just means they don't know what species it is. Some live attached to rocks, some live quite nicely in floating masses. The Sargasso Sea is a mass of floating Sargassum.
Care for most species is pretty straightforward as long as there is enough light. It seems to do well under standard reef tank conditions, with low nutrients and good water quality. Once established, it can grow fast, many species grow large, and few herbivores eat it, so it can become a bit of a pest.
Where did the stuff come from? If you post a pic, I can try to key it out for you.