Malcolm is Ursula's stalker in "The One With The Jam", who meets Phoebe shortly after mistaking her for Ursula and consequently beginning to stalk her, appearing to be embarrassed when she confronts him and informs him that she is Ursula's twin sister. It quickly becomes clear that Malcolm has an unhealthy obsession with Ursula which is making him "love drunk," but Phoebe seemingly helps him get over this and starts dating him - until she learns that he actually isn't cured of his obsession after all and is still stalking Ursula. Phoebe ends it with Malcolm because it is too weird but offers to help him get over Ursula. She even suggests that he stalk her for a while - which he immediately does.
Although Ursula made further appearances in the show, Malcolm was never seen again after this episode. From this it is safe to assume that Malcolm eventually did get over his obsession with Ursula and stop stalking her.