Al-khalūq, with an open kh sound, is a composite fragrance prepared from saffron and other perfumes.

The ḥadith indicates that the mosque should be cleaned, kept free from anything repulsive, and perfumed with al-khalūq or any other scent. Ibn ʿAbd al-Barr said:
“Just as spitting in the mosque is prohibited, so too must it be kept free from eating things such as acorns or raisins, because of their pits, and from anything greasy or messy, grains of flour, and whatever a person sweeps out of his house.”
Al-ʿIrāqī cited this in Sharḥ al-Taqrīb (2/385).

For the same reason, it is not permissible to throw in the mosque any insects—or anything comparable that one removes from his body—because the Messenger of Allah (Allah bless him and grant him peace) said: “If any of you finds a louse in his garment, let him crush it and not cast it into the mosque.”

Aḥmad recorded the ḥadith (5/410): Ismāʿīl narrated to us; Ḥajjāj al-Ṣawwāf narrated to me from Yaḥyā ibn Abī Kathīr, from al-Ḥaḍramī ibn Lāḥiq, from a man of the Anṣār, that the Messenger of Allah (Allah bless him and grant him peace) said… and so on.