Complexes are often used for this situation. Ideally they should be sister species and referred to as complexes in literature. If they’re nearly identical and hybridize regularly then that sounds appropriate, but with a quick search I didn’t find any instances of them being called a complex.
In the mean time there are only 8 species in Elodea so pushing back to genus isn’t as bad in terms to reducing the meaningfulness of the ID as it would be in some large genera.