Marcy.....Depending upon the current status of the current situations of these 2 individuals....for instance, if you have known one another casually for some time, exchanged conversation, interacted separately in the same events or functions.....then it seems to me you are familiar with one another enough, to have at least "taken notice" of one another.
Within the scenario above or one similar, I think the contents of one's heart should take the lead. It's clear the mere thought of sending a card has some sense of purpose. I would follow it.
It's only up to us to act upon our feelings and thoughts with sincerity. We can't allow ourselves to wonder or predict the reaction of the receiver. This is entirely their own personal choice. (it could be that this person has some of the same feelings, but it also dealing with apprehension)
The choice of the card should probably be selective in terms of the greeting. You may want to choose a card with a kind and generic sort of expression like one you might send to any friend. This can only be defined as a kind, sweet gesture and give you no cause to be concerned. On the other hand, it could open up the doors to further "interest" and allow HIM to make the next move.......and while it may not be immediate, it may be worth it...........:) My belief has always been to follow my heart. You can't go wrong with that, regardless of the outcome. In any event, at least you will have made your subtle effort.....