From the Super Mario Wiki, the Mario encyclopedia
Shroob Mushrooms are light-purple Mushrooms with white dots that appear during the events of Mario & Luigi: Partners in Time. The Shroobs have the ability to fire purple energy bolts that turn anything they touch into Shroob Mushrooms in a process; Toadiko, Toadbert, Kylie Koopa and Baby Bowser are all known to suffer this fate. People who turn into a Shroob Mushroom are said to be Shroobified. These victims are all reverted when Professor E. Gadd spreads the tears of babies from the Hydrogush 4000 across the Mushroom Kingdom (in a process that Stuffwell describes as being able to "re-Toadify" them, even if not all of them are Toads). Some Shroobs are also known to turn into these purple mushrooms when defeated, like Elder Princess Shroob. When Princess Shroob is disguised as Princess Peach, she holds a Shroob Mushroom to her face. Shroob Mushrooms have been rarely able to speak, such as after Elder Princess Shroob gets defeated.