Molder is the American spelling of British moulder. As a verb it means to fall apart slowly — wood, paper, old cloth, a house left in the rain. It is not a sudden smash. It is a quiet rotting or crumbling, often over years: “The books moldered in the damp cellar.” You will meet this more in writing than in speech, and it feels a little old and bookish. People also say molder away. As a noun, a molder can be a person whose job is to shape things in a mold, for example in a metal shop. So the same American spelling does two jobs: slow decay, and “someone who molds.” Those two sit on different sides of mold — the rotting side and the shape side — though actually everyday talk uses the verb of decay more than the job-name.