@timetravelingcat
Wouldn't need a mouth =/= Wouldn't have a mouth
I imagine one or more of the following is true
A) It's vestigial
B) It's still used for communication
C) They still eat non-airborne food when it is available for the additional nutrition (this definitely seems to be the case - hence why eating helped her when she was sick)
Tons of species possess features that aren't strictly necessary for their survival, either because they may provide some supplemental/situational benefit or because there was no particular reason to lose the feature (vestigial).
And disregarding all of this, just google filter feeders and see how many have mouths. You can't really say for certain what a hypothetical human-sized microbe filter feeder would look like without knowing the particulars of their eating mechanics or their evolutionary lineage.
And yes aliens don't HAVE to look humanoid. It also might be interesting to see more interpretations where they aren't. But this is essentially just a monster girl manga, just accept it for what it is.