Fun fact: Canonically Ducceldius does not have a mouth, nose, or face for that matter (not counting his sunglasses). Meaning if he wasn't glowing, he would look like slenderman, or one of those photoshopped photos of faceless people.
In order to eat, he has to melt the area where his mouth WOULD be (since he's a non-newtonian fluid)
The reason he can talk is because his body makes vibrations that sound like speech. The same works for speech.