Got a picture? Just from your description its hard to be sure but it could be a number of things....
In newer tanks fungus is not uncommon. It is usually white in appearance as is your mystery substance, but more often than not it forms on the substrate, not the plant leaves.
Most algae turns white as it dies. It might have been staghorn which is just more noticeable now that it dying off. This could be a result of the addition of C02 or a preferable shift of other tank parameters.
If you did just add the C02 however, it might be another culprit... The Co2 reactor itself. If you left the tube to long or the 2 liter itself has tipped over it is possible for the yeast/sugar/water mixture to back siphon into the tank. When it does. it usually results in a slimy, white substance that will cling to your plants and hardscape.
These are just a few possibilities, I am sure there are others. If you can add a picture and give us some additional info I'm sure someone will be able to tell you for sure.:thumbsup:
