Haze is a loss of optical clarity caused by light scattering at the surface or within the sheet. Several mechanisms can look similar, so the defect should not be assigned to moisture without evidence.
Internal bubbles, microvoids, crystallization in susceptible polymers, thermal degradation, and residual stress can create internal haze. Surface abrasion, tool texture, incomplete contact, masking damage, or chemical attack create a surface-based effect.
Separate surface from bulk
The part should be inspected from both sides and compared with an unformed sheet from the same batch. A surface defect may change appearance when wetted temporarily with a compatible optical fluid; a bulk defect will remain. Microscopy or a controlled polish on scrap can provide further separation.
Correction depends on the material. Drying, shorter heat exposure, faster cooling, higher forming temperature, or a different tool finish may each be appropriate for a different mechanism. A single generic “clear plastic” recipe is not reliable.