Maybe it's a reflection of the coralline but the tips of the polyps look a bit pink to me. I see where the base has a faint hint of green so I may be mistaken on its original colour.
The shorter polyps on a Goniopora sp., in my experience, usually are a result of the coral dying.
When they first come into captivity, they usually are very long and flowy. Then after about 6 months, they stop opening up fully. Then they get a 'bad spot' which usually spreads until all that's left are some stubby tentacles and a lot of exposed skeleton, then eventually they wither and die.
With the advent of better foods for fine filter feeders, their survival rate is somewhat better, but by and large they aren't an easy coral to keep.
Most are a deep green, or even red. That yellow one in the picture I've never seen before, but I didn't bring them in (I'd get one as an occasional 'filler' and then I'd blow my supplier's hair back for sending it to me.)
That yellow one in the picture already has a bad spot on the bottom left. Not sure if that's a naturally occurring colour or not.
Keep in mind too, that some folks dye corals. This is something that I consider deplorable. It doesn't occur as often as it used to, but if you chttp://www.reefkeeping.com/issues/2002-09/eb/">heck this article</a>, you'll see a Goniopora dyed yellow. Quite possible that the specimen in the picture (the yellow one) is dyed.
Dyed corals usually either die, or their colours wash out over time.
Dyed corals are a no-no in my book.
Jenn
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