Cleaning butyl rubber
According to this MSDS, Astlett Rubber, Inc.'s BUTYL RUBBER BK-1675N product contains multiple stabilizers and antiaglomerates, each of which is described as a white powder. I suspect it is the leaching or other means of migration of these stabilizer's to the rubber surface that causes the white spots. The following is an excerpt from the above MSDS:  


  Butyl rubber contains moisture, stabilizer (Agidol-2 or Irganox-1010)
  and antiaglomerate (calcium stearate or zinc stearate). Stabilizer and
  antiaglomerate are not discharged into the atmosphere during
  processing butyl rubber into finished goods.  
  
  Stabilizer Agidol-2 is a white crystalline combustible powder. Flash
  point in the open crucible is 172°C, ignition point is 208°C, the
  standard temperature of spontaneous ignition is 352°C. Agidol-2 is
  permitted for use as a stabilizer of rubber that is used in food
  industry, in water  pipelines, and children toys production.  
  
  Stabilizer Irganox-1010 is white crystalline combustible powder.
  Ignition point is 288°C. Meets all requirements for use as a
  stabilizer in rubber th at is used in medicine.  
  
  Zinc Stearate is white crystalline powder or paste. The standard
  temperature of spontaneous ignition is 900°C. Maximum permissible
  concentration in work area is 4 mg/m3.  
  
  Calcium Stearate is a white powder. The standard temperature of
  spontaneous ignition is 560°C. Maximum permissible concentration in
  work area  is 10mg/m3.