That "cloud" must reflect the thoughts and memories of anyone inside, creating an illusion and warping the sense of perception.
The two "robots" called it a "creeper", so there is a chance it is some sort of living being, but it could be just an expression of the force field mixed with atmospheric elements, or could even be a cloud of microorganisms, like plankton.
This being the Abyss, it could even be all the options above.
I am always joyful when I see that the translator of this series keeps the artful SFX translations going.