Hundreds of these in Lake Merritt right now. Well, at least 100, could be more. We only checked the area around the channel but there were plenty in the channel and in the lake around the channel. Many seemed to be eating the Ulva, others seemed to be forming big clusters like the second pic. Is that mating behavior, and are those eggs in the middle? We drew a small crowd of onlookers, so we did some 'splainin'. None of them inked us. The slugs, not the onlookers.

Thanks to Joel Peter for the lead.