Ghadāʾir is the plural of ghadīrah, a lock or braid. Its suffix -hu refers back to the previously mentioned farʿ, the hair. In ghadāʾiruhu mustashzirātun, the first phrase is the subject of a nominal clause and the second its predicate.
Mustashzirāt is a sound feminine plural form, presenting multiple raised sections of hair. Ilā al-ʿulā specifies their upward direction.
The following imperfect taḍillu introduces an active, ongoing impression: the bound locks become hard to follow. The nominal clause establishes the arrangement, and the imperfect verb makes the image more vivid.