Most starfish eat mollusks. Some eat sponges or plankton. The Crown of Thorns starfish (COTs for short or Alamea in Samoan) actually eat the reef.  They are for some reason out-of-balance in the ecosystem at outbreak levels and are destroying the reef in many areas.  There has been an ongoing effort by several agencies here to kill them and preserve the reef.  They have a very painful sting if you touch them!  My neighbor Kelley goes out regularly to remove them from the reef in front of our houses at Coconut Point.  This day we were actually just paddling and snorkeling, but Sabrina found this large COT and we felt the need to remove him.  The coral dies and turns white in the areas where the COT has been attacking it, so generally you find them by looking for white (dead) areas in the reef.