SB 5.20.13
evam surodad bahis tad-dvi-gunah samanenavrto ghrtodena yatha-purvah kusa-dvipo yasmin kusa-
stambo deva-krtas tad-dvipakhyakaro jvalana ivaparah sva-saspa-rocisa diso virajayati.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Outside the ocean of liquor is another island, known as Kusadvipa, which is 800,000 yojanas [6,400,000 miles] wide, twice as wide as the ocean of liquor. As Salmalidvipa is surrounded by a liquor ocean, Kusadvipa is surrounded by an ocean of liquid ghee as broad as the island itself. On Kusadvipa there are clumps of kusa grass, from which the island takes its name. This kusa grass, which was created by the demigods by the will of the Supreme Lord, appears like a second form of fire, but with very mild and pleasing flames. Its young shoots illuminate all directions.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
From the descriptions in this verse, we can make an educated guess about the nature of the flames on the moon. Like the sun, the moon must also be full of flames because without flames there cannot be illumination. The flames on the moon, however, unlike those on the sun, must be mild and pleasing. This is our conviction. The modern theory that the moon is full of dust is not accepted in the verses of Srimad-Bhagavatam. In regard to this verse, Srila Visvanatha Cakravarti Thakura says, susaspani sukomala-sikhas tesam rocisa: the kusa grass illuminates all directions, but its flames are very mild and pleasing. This gives some idea of the flames existing on the moon.