I believe there are two types of "what is that"
그게 뭐야? and 그건 뭐야? right?
And any real difference between the two?
그것이→그거이→그게 (still difficult for me)
그게/<sub>이/</sub>가 is concerned with the thing, 그건/<sub>은/</sub>는 is focused on its type or some related topic.
Is the contraction 그게 specific to 해채?
그게 뭔데? is used more in kdramas.
그게 뭔데 also means "what is that?" And can be used while talking instead of 그게 뭐야.
Apparently 그게 뭔데? maps to "What's that about?"
(That sense of ~데 is missing from Wiktionary. It must be that connector like in Japanese. Here in northern Kyūshū people even use it by itself. "And (so) . . . ?" "And what?")