Plant it to grow Aqua Cave Crystal. These crystals grow naturally in caves. They don't actually glow - bioluminescent organisms feed on them and emit light. Also heat - be careful!
Aqua Cave Crystal Seed price in Growtopia: sells for about 1 WL, buyers pay about 5/WL (median of 9 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.
Recently sold at: HEDCRIME1, NMKSHOP