Dry ice is just solid CO2 which is why it is so cold. When it sits in air, there is no reaction, as it is simply melting. The by product of the sublimation (solid -> gas) is gaseous CO2. This is naturally found in your reef water but in excess it will lower your PH.
On the other hand, dropping it in water will actually have a chemical reaction and creates an acid (can’t remember the name). This acid should remain in the container, but you definitely don’t want this finding its way into your tank.
Also note what nautical_nathaniel said - you need to give the CO2 somewhere to go. It is denser than air so it will pool on the floor of the room. If not properly ventilated, it can create high levels and be very dangerous to people.
