I can't guess what that second character is on the geoduck. (WHY is there a geoduck?!)
For some reason, this artist draws geoducks a lot. As in almost every single post tagged with geoduck is by him. It seems to be his artist self-insert, judging by his Pixiv profile picture.
This artist's name on pixiv and twitter is ミル貝の人, and his twitter plaintext handle "mirugai_human" is a literal translation of the same.
Mirugai refers to the mirugai clam, Tresus keenae. While similar, mirugai clams and geoducks are not the same thing; the latter species is Panopea generosa. So translating ミル貝のイラスト as "Geoduck's illustrations" in the commentary, or using the geoduck tag with this artist is possibly incorrect.
With that said, it seems that the Japanese themselves might fail to make the distinction. According to the Wikipedia article on geoducks:
In Japan, geoduck is prepared as raw sashimi, dipped in soy sauce and wasabi. On Japanese menus in cheaper sushi restaurants, geoduck is sometimes substituted for Tresus keenae, a species of horse clam, and labeled mirugai or mirukuigai.