What is the symbolic meaning of " mould " in the poem?

Mould or, as the Pearl Poet calls it in his dialect of Middle English, "moul," has a meaning both literal and figurative. This mould, or "clot," is literally imagined as dirt coating, and therefore marring and dirtying, a pearl. However, in the context of the poem, the pearl is a representation of the speaker's lost daughter.It is a pure symbol signifying a dead female child. As such, the mould or "clot" is representative of grave dirt and, by further analogy, death itself. Where previously the pearl was "wythouten spotte," or without any blemishes, now it is "clad in clot," or deteriorated by dirt.
The implications of this are slightly macabre—note the reference to "rot" in line 26. The poet is here describing the natural process by which plants break down, but its closeness to the comments about the pearl being covered by mould directs the reader to imagine how a "pearl" might be deteriorated by grave dirt. The pearl is a "jewel," but the earth breaks it (her) down through the natural process of decay.