As nouns the difference between hydroid and sertularian
is that
hydroid is any of many colonial coelenterates that exist mainly as a polyp; a hydrozoan while
sertularian is any species of genus genus: Sertularella, or of family
family: Sertulariidae, hydroids having branched chitinous stems and simple sessile hydrothecae.
As an adjective hydroid
is of or pertaining to such creatures.