This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
萏 combines 艹 (grass/plant) with 臽, but the exact historical development and original meaning are uncertain.
The top 艹 suggests a plant, and the bottom 臽 looks like a pit or trap. Picture a plant growing over a hidden pit—its meaning is unclear, but it might relate to something floral or concealed.
For タン, imagine a tan-colored plant (艹) spreading over a pit (臽): tan -> タン.