The "fighting starfish" is when one individual rubs their anus against the anus of a second (consenting) individual. It is similiar to the act of "scissoring"- which typically occurs between two vaginas. The "fighting starfish" is more inclusive than scissoring because anyone can participate so long as they have an anus.
Person 1: "What did you do on your date last night?"
Person 2: "Well, Stan and I tried out the fighting starfish..."
Person 1: "Whats that?"
Person 2: "It is when two consenting adults rub their anuses together. It is like scissoring, but more inclusive."
Person 1: "Oh, that sounds like fun."