From what I've read (I don't own any), Caulerpa tends to go sexual when it gets crowded as a self preservation thing (too dense for light to penetrate through it, etc). As long as the fish are keeping it pruned you should be ok. I think it starts to change (white strikes my mind) when it is ready to go sexual on you. You could always switch over to chaeto if the thought of it going sexual in your tank makes you nervous though.
I'll be putting chaeto in my fuge. Too chicken to run with Caulerpa.
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