open brain colour/ zoo?

[Delphinus]

Hmmm, that's odd. I wouldn't discount the galaxy but the thing is if the brain had gotten stung, it's not likely to expand the tissue in the affected area (i.e., more likely it would pull in tight). But the tissue does look a little bleached, so something's up. It could be that it just shed some zooxanthellae, which does happen (and isn't necessarily fatal unless it's a bad stress response, but sometimes it just happens -- usually in response to a change in light intensity or maybe even a temperature swing - in those cases, if all other things are good then it should be able to recover.)