This is exactly the kind of card idea I like. It's simple, yet it offers many cool possibilities. The choice of element though.. like TakoMan02 said,
might fit better, especially since fire is considered a rush element, and you could now try to play your cards as fast as possible, while at the same time try to slow down your opponent. If this was "decay", it should mean something like having to discard 2 cards after 3 rounds or something.
I would also make one small change to the art. Maybe add some cracks in the card or something to signify decay? But that is just cosmetic change.
Other than that, I really like the idea.