This is the "flamingo tongue snail," one of the most brightly coloured snails on the reefs of the South Atlantic. It used to be a common sight, but in recent years has been disappearing, either because fate has a cruel sense of humour or people are uneducated jerks. Or both.
The flamingo tongue snail spends much of its days munching on coral. If you see it on a piece of coral, you would do well to leave that piece of coral alone. The snail is famous for singling out poisonous coral, which it can eat with no ill effects. It takes advantage of its diet by saving the poisons brewed by the coral and keeping them in its own body, making it just as dangerous to predators as the coral it ingests.
Its toxicity is why it can cover its body with the gaudiest possible combination of colours, have the low speed of a snail, and largely remain unharmed. It does have a few predators, and in the past over-fishing of those predators has caused the snail population to go up, and the coral population to go way down.
According to the Department of Defence Legacy Resource Management program, flamingo tongued snail numbers are declining. Shell collectors and scuba divers often pick up the flamingo tongue snail as a souvenir of their trip. What was once a sign of the snail's invulnerability has now made it vulnerable. The snails are slowly disappearing from common snorkelling and scuba diving spots because people want their colourful shells.
Which brings us to the final irony: the brilliant spots are not part of their shells. Their shells are quite boring. Observe:
The spots are on the snail's mantle, a fleshy expanse which it can stretch out over its shell most of the day, and pull back in if it's ever attacked. The mantle stays out most of the time because it contains the snail's gills. When the snail gets taken out of the water, it dies, and the mantle shrivels up, leaving the shell-collector with a boring yellow-beige shell to remind them of what a selfish jerk they are.