Colt corals aren't considered leathers. I'm a beginner and my first coral was a colt coral. Very pretty and easy to care for. Mine grows under PC lighting. It can grow fast, but not with the speed that Green Star Polyps grow. Those things can over run your tank in a matter of months depending on the size of your tank. Mushrooms like Eric765 mentioned, are very easy to care for. They don't need to be fed b/c they're photosynthetic, but you can feed them. Green Star Polyps are very easy to care for and are very pretty. They spread VERY FAST. Zoanthids are also easy to care for. Xenia and leathers are also easy.