Update: Thanks for the advice but I wasn't confident in my plaster skills with 0 experience. Called out a pool plaster repair man recommended by a coworker. $300 later it's fixed. He completely cut out that piece and patched with new plaster. Also said the plaster looks fine for it's age, it's just a failure. They happen sometimes, not a sign of any huge issue. Hopefully I'm good for a few years or more.