Lunar vesicular basalt 15556 contains evidence of volatiles in the form of gas bubbles. We report two occurrences of apatite needle mats, forming flow patterns from the rock interior onto vesicle walls. The mats on vesicle walls are free of silicate minerals or glass. The apatite needles display aspect ratios (length to width) of 8 to 21, implying the presence of water vapor as shown by available experimental results. Although all apatites are fluorapatite, those on the wall contain lower F (higher inferred OH) and higher rare earth elements than nearly pure fluorapatite away from vesicles, indicating apatite needles in these mats formed from highly evolved water-rich melt. Collectively, the morphology, texture, and chemistry of the fluorapatite needles on vesicle walls support that these crystals grew rapidly and out-of-equilibrium from a final-stage liquid that changed from water-bearing silicate-rich to silicate-poor water-rich during its flow from interior to the vesicle wall.