I say, with Allah's guidance: It is disliked to clean with grass because it is fodder for animals and because it defiles the pure without necessity. However, if one does clean with it, it is permissible but disliked, because the important factor is cleanliness, which is achieved; and because the prohibition is due to a reason unrelated to it, it does not prevent achieving purity: like cleaning with someone else's cloth or water. See: Bada'i al-Sana'i, 1/18, Radd al-Muhtar, 1/339, Al-Ikhtiyar, 1/48-49, Al-Lubab, 1/46, Al-Hadiyah Al-Alaiyah, p. 45, and Allah knows best.