Problem with my saltwater fish tank and clownfish?

Hi everybody! I have had a 15 gallon saltwater tank with two clownfish for 6 months. I just cleaned the tank 4 days ago and this purple, stringy thing has developed in my tank along with some light green spots. Also for some reason, my clownfish are starting to twitch sometimes for some reason and it seems like they are always picking at each other. What should I do?