The prestige class thrall to Juiblex gains the supernatural ability polymorph self that says, "An 8th-level thrall of Juiblex’s form is more amorphous than ever. It can change its appearance and form at will, as the polymorph self spell cast by an 8th-level caster" (Book of Vile Darkness 70). (The DM is updating polymorph self to polymorph to bring the thrall in line with the 3.5 revision.)
Absent that caster level, this supernatural ability's caster level would be the thrall's Hit Dice, but, because the ability's caster level is otherwise noted, the ability's caster level is fixed at 8… seemingly forevermore.
Before I recommend a new player takes on this character as a PC, I want to confirm: Are there any game elements that can raise this—or, more generally, any—supernatural ability's fixed caster level? Or is this something the DM needn't worry about?
Note: For raising the fixed caster level of a spell-like ability, see answers to this question. By the way, yes, Mold is a warforged thrall to Juiblex who's reformed and now uses his unlimited ooze-summoning and form-changing abilities to fight crime. The Saturday morning cartoon almost writes itself.