The OP authors tell us (p.1),
A new type of habitable exoplanet within the sub-Neptune regime was recently proposed by Madhusudhan et al. (2021), known as Hycean worlds.
Madhusudhan et al. 2021 [0] tell us (p.3),
... in this study we focus on planets that allow for large oceans with habitable conditions underneath H2 -rich atmospheres. We refer to such planets as "Hycean" worlds.
I guess 'hydrogen' + 'ocean' = 'hycean'. There might be other explanations in Madhusudhan but I'm not reading the entire 25 pages to find them.
The capitalization, used by both sources, is puzzling: is it named after a person or place? A sci-fi author, perhaps? Otherwise, it should be lowercase. Also, anyone inventing a word should be required to grace us with a pronunciation.
What is the point of peer review if they don't catch these issues? ;)
[0] https://iopscience.iop.org/article/10.3847/1538-4357/abfd9c