I’m not certain whether the unusual SW winter swells and storms of 2021/2022 have contributed to an abundance of this species in certain areas, or whether upturned stones and broken dead coral have exposed this relatively uncommon species in unusual numbers, but these 15-25 mm animals were found in relative abundance off of Olowalu, Maui in very shallow rubble at low tide. It can be very difficult for the novice eye to tell this species apart from Mourgona Sp. #4.  The differences are subtle and only close examination and perhaps expert consultation will enable one to tell the difference. I rely on Cory Pittman and the Sea Slugs of Hawaii site, which goes into much more detail.  Primarily, Mourgona sp. #1 has noticeable spots/markings behind the eyes.  If you can’t get a clear photograph of that part of the slug, then the other major tell-tale difference is that # 1 lacks the distinct purple patches that are visible within the tips of the cerata of Mourgona sp. #4.