cor·po·re·al
kôrˈpôrēəl/Submit
adjective
relating to a person's body, especially as opposed to their spirit.
"he was frank about his corporeal appetites"
having a body.
"a corporeal God"
synonyms: bodily, fleshly, carnal, corporal, somatic, human, mortal, earthly, physical, material, tangible, concrete, real, actual, this-worldly
"they tried to bring Satan into corporeal existence"
Definition of corporeal
1 : having, consisting of, or relating to a physical material body: such as
a : not spiritual
… some few traces of a diviner nature which look out through his corporeal baseness …
—Robert Browning
b : not immaterial or intangible : SUBSTANTIAL
corporeal property
By definition molecules are tangible. The device you access this website with is comprised of molecules. You can touch it. It's physical, tangible, corporeal. It's a physical thing. Molecules are physical things. You feel the wind, you smell the scents.
A gaseous object or being would be corporeal. Gas is made of molecules.
In contrast if something is non-corporeal then it's not tangible. It's not physical.
So the problem comes down to "are Medusans, Organians, etc.. corporeal or non-corporeal?" If they are non-corporeal, then they, by definition, are not made up of molecules. It's the definition of the word non-corporeal.
Now, they may be comprised of some physical or tangible property we have not discovered yet. X-rays, gamma rays, etc.. have existed since the universe began yet for the majority of human history those particles were unknown and undetectable by any means. So, if a Medusian is a corporeal being composed of something that, while real, our science (or the science of the time) cannot define or explain, then maybe they are at risk to the Borg.
Q being scared of the Borg though? I don't seem to recall that ever happening. Between being able to travel in time, disappear at whim, actually give other beings advanced powers and abilities, or shrink objects to the size of a keychain, I don't see where the Borg are much of a threat.