[amoeba]

villains underbet on the flop does look like a flush draw.

alternatively you could try something like raise to $12 on the flop (you are still charging him incorrect odds for the draw as its $9 to call in to a $35 pot for a 1 card flush).

then pot on turn is $44, you bet 1/2 pot.

I don't do this very often but from a theory perspective without regard for metagame it makes sense.