These plain white sheets have become a symbol for the scale of death amid Israel's bombardment of the Palestinian enclave.
But Mussa, who volunteers for the Keratan Society, which prepares dead bodies for burial, says they ran out in the first days of the war.
He was, he says, forced to wrap four or five bodies in a single covering.
"I never expected this number of deaths, at all. We did not have such numbers in the previous wars. You are talking about unimaginable numbers; you are talking about over 20,000 martyrs not including the people missing."
While beseiged Gaza faces severe shortages of food, water and medicine, shrouds have since been in abundant supply, thanks to donations from Arab countries and charities.
But Mussa says there are other difficulties in the blockaded territory, such as a shortage of scissors, knives and cotton needed to prepare the coverings.
Israel began bombarding Gaza in retaliation for Hamas killing 1,200 people and taking 240 hostages in its October 7 cross-border raid.
Israel's military has expressed regret for civilian deaths and blamed Hamas for operating in densely populated areas.
The importance of the shrouds stems from a narration by the Prophet Muhammad, who encouraged his followers to wear white clothes and also wrap their dead in white.
Marwan Al-Hams, director of the Abu Yousef Al Najjar hospital in Rafah, says the shrouds have also gained new significance as Gaza's death toll has risen.
"The big number of martyrs made the white shroud a symbol for this war and it became parallel to the Palestine flag in its influence and the knowledge of the world about the significance of our cause."
For those who have lost loved ones, the shrouds are also a place to write a last heartfelt message.
But not all coverings can carry such words.
Such is the war's chaos, some of the dead cannot be immediately identified.
In those cases, "unknown male" or "unknown female" is written on the shroud amid hopes they can be identified at a later date.
