Quite a few of you guessed this one correctly. It's a detail seen from the queue for Finding Nemo Submarine Voyage, and the TL59 stands for Tomorrowland and 1959 - the year the original Submarine Voyage attraction opened.
Submarine Voyage opened at a time when nuclear subs were in the news and seen as the future of technology. At the time, one of the subs in the fleet was christened Nautilus after the USS Nautilus. Launched in 1954, the USS Nautilus was the first nuclear-powered submarine and became the first to cross the North Pole (a story point incorporated into the ride).
Following their naval inspiration, the subs at first were painted a military grey. This changed in the 1980s, as the story was adjusted slightly to focus more on marine research and the subs were painted yellow.
The original Submarine Voyage took its last dive in 1998, the victim of ever-increasing maintenance costs, but a strong desire remained to revive the attraction in some way. With the Pixar film Finding Nemo as a motivator for a new story line, the Submarine Voyage finally returned in 2007.
To solve some of the challenges previously experienced with the attraction, some of the show pieces are actually presented behind glass or via other special effects. This still creates the illusion that they're underwater, but protects the scenes from the damage caused by being submerged. In the case of the coral, it was painted using a new technique in which colored glass was incorporated into the paint, allowing it to hold its color exponentially longer.
Now, thanks to the creativity and problem-solving skills of the Imagineers, guests at Disneyland can once again enjoy the thrill of diving to the ocean's depths -- just as they did back in 1959.