Notes:

It's honestly hard to tell what kind of creature it is… But looking at the thing in profile makes the Stevenesque facial features even more apparent. Dear God it IS him isn't it? And since White Diamond is known to still have a trace of Pink off-color in her, that's probably how he's controlling her.

PS this image happens to have a very good view of "Pietersite" – it's obvious that her chest, stomach, arms and left legs are all fully clothed, meaning she's got no gems there. Pretty much confirmed for EyeballXAquamarine fusion.