Will RPE'sattack their neighbor?

[crypul]

I have a good sized colony of RPE's. It is placed pretty close to a colony of yellow polyps which, up until recently, were very healthy. Now a large portion of the yellow polyps closest to the RPE's have changed to a lighter yellow color and their skirts have shortened quite a bit. They also close up for periods during the day but never did that before. I never thought of RPE's as aggressive but could they be the cause?

 

[650-IS350]

Yellow actually grow like weeds... the only way zoa's and palys can kill each other is out grow and shade the opponent, no light means starving the enemy out.

 

[crypul]

Yeah I know that the yellow can grow crazy but these are in a controlled area and I like the way they flow in the current. They have me stumped though. They are not blocked for light still eat when I feed. Any other ideas what could be causing it?