Taghlibī is a nisbah: the suffix connects an individual to the tribe of Taghlib. Maṭlūl has the pattern of a passive participle and describes blood left without compensation. One word names affiliation; the other names a resulting condition.
In mā aṣābū min Taghlibiyyin, mā introduces a general account of those affected and min Taghlibiyyin identifies the tribal affiliation. Fa-maṭlūlun gives the outcome the poet asserts.