The thing is there is sometimes a little confusion in the word morph for zoanthids. Environmental conditions will get zoanthids to "morph" but generally will not produce an actually different permanent change or "morph". On the other hand over time there can be subtle change\mutation in the cloning process or simple sexual reproduction variation. Another instance of change can be from loss of zoozanthellae but this usually can return over time or just makes for a less attractive polyp. Several corals have the ability to re capture alternative zoozanthellae so I might believe this explanation for zoanthids and palythoas under certain circumstances but have yet to see any documented evidence supporting it for this genus.
Generally speaking less light brings out the polyp itself giving it a stretch look or vice versa for higher light, they can form a low mat over the substrate.
Less flow can also have effect not only on the base length but also the skirts, punk rockers, armor of the gods, and armagedons , for exampe, get a much more frilly skirt with moderate to low flow.
Nutrient loads can cause coloration changes in some zoanthids too. Many will suffer a lack of vibrance when the nitrate or phosphate levels are too low or zero. If you have a variety that eats well you can supplement them this way as much can be derived from the proper foods.
Last but certainly not least is lighting. We all know corals look different in our tanks Vs the next guys tank, or the vendor we purchase from for that matter. So first off is the initial spectrum being a culprit of appearance. Then there is the increased or decreased spectrum and or light intensity. This causes various zoozanthellae to fade away or to become dominant over another. Translation is a change or "morph" in colors. Most often this is lost once the zoanthid is returned to a previous lighting regime as the varying types of symbiotic algae still reside within the tissue and just need the proper conditions to return.