After you change your clothes for the theater play to the Ariel character the graffiti is right there. You stand at the left mirror and the right mirror beside you is the graffiti, so go there and interact with the mirror.
00:08 - Optional Graffiti #1 (Stagehandwriting)
00:44 - Optional Graffiti #2 (Radical Piratical)
01:39 - Optional Graffiti #3 (Feels on Wheels)
02:17 - Optional Graffiti #4 (Canon Wall)
02:45 - Optional Graffiti #5 (Wishlist)
03:23 - Optional Graffiti #7 (Tread Harshly)
03:53 - Optional Graffiti #6 (Friendly Forest Friends)
07:12 - Optional Graffiti #8 (Permanent Record)
08:38 - Optional Graffiti #9 (Vanity Fare)09:21 - Optional Graffiti #10 (Creature Feature)