كَالْفَرَاشِ

كَالْفَرَاشِ is a phrase (شِبْهُ الجُمْلَةِ), a compound consisting of a particle of pulling down (preposition) and a genitive noun

الْفَرَاشُ is the natural state of this definite noun and it changes to كَالْفَرَاشِ due to the preposition كَ that precedes it

PhraseParticle/NounNoun Particle 
شِبْهُ الجُمْلَةِجَارٌ وَمَجْرُوْرٌاسْمٌ مَجْرُوْرٌحَرْفُ جَرٍّ
PhraseCompoundGenitive NounParticle of pulling down
  
like mothكَالْفَرَاشِ=الْفَرَاشِ+كَ
like mothmothlike
Surah/Below is the only verse in the holy Quran containing the genitive particle and genitive noun compound كَالْفَرَاشِ
Verse
101-4it is a day when the people will be like scattered mothيَوْمَ يَكُوْنُ النَّاسُ كَالْفَرَاشِ الْمَبْثُوْثِ