That sounds like the sandcastle CG. To get it you must be raising the romance/melting level of both Liam and Mick and then meet both of them at the beach on a weekend before the route choice. A prerequisite is to have already met Mick at the beach.
That's a missable CG related to the spoiler character. You need to go the park when he's about 4-5 parts melted. There might be some Garret or Mick scenes that have higher priority but once you've done them, you should see his symbol there if the requirements are complete. (If you need further help, check out the guide for the only scene spoiler gets at the park)