There is no organized clergy in Islam, so they informally refer to themselves as Imam or Alim (basically calling themselves scholars or learned men). Many areas have evolved their own religious heirarchy, such as the Taliban or the Ayllatolahs, but Islam by doctrine isn't supposed to acknowledge that any man knows God any better than the next.
The term preacher is vague and informal enough to be a pretty decent translation, especially because these guys tend to lean towards the 'fire and brimstone' angle.