You're interested in your daughter's casual friend with benefits?
Let it go. Like right now.
There is no way for this to develop right now that does not hurt your daughter. None. Even if he is interested - which is possible - the possible damage to your relationship with your daughter makes this a no-go. It doesn't matter how casual a relationship it is.
Unless you are close enough to your daughter to say, I am attracted to your casual FWB, he seems interested in me, your father and I have an open relationship and he is ok with this. I would like to date so and so. Is that ok with you?
(As an aside, is your husband ok with this? He's not worried about your daughter?)
I don't know many parent-child relationships close enough and mature enough to handle this without massive stress and damage. And your daughter is only 19 - an adult but a brand spanking new one. Don't put this stress on her. It's not right.
MAYBE if they stop seeing each other, and a few years go by... Then you can revisit this crush. But this is not worth potentially seriously damaging your relationship with your daughter.