ive had zoas grow long under plenty of light. I kind of had the forest effect going on. they ran out of space to spread out so started growing up and shading each other out which caused the ones next to them to grow taller and it was like a race. now pretty much all of them in my tank are like this.
before I switched out my 250 watt halides for leds I had zoas growing with long stalks about 8 inches from the water surface directly under the halide. So there can be other factors causing it to grow up as well.
Although in your case being that it seems they have plenty of room to spread yet they grew up, it seems like they may be needing more light.