Not before Factorio 1.0, if it's possible at all. To be honest, I've no idea yet how to make that work. Trees are relatively easy because they are a vanilla concept. But algae? Yes, there's greenish water, but I guess it has to be placed during map generation -- not sure you can replace water later on. There should also be multiple shades of green to indicate differences in algae population, and color transitions between clear blue water and water with algae. Perhaps it could work with defining new tiles, but tiles are a rare commodity (there are only 255 or so available, and if you have mods like Alien Biomes or Dectorio installed, there are few left for other mods).
If you have any suggestions how to deal with these problems, I might consider adding this in the future. In fact, it would open new techs (e.g., make it necessary to clean the water before using it in boilers etc.). Then again, new machines would be needed to process the water, meaning new graphics and animations would have to be made. (But I'm not an artist, something like that is beyond my abilities.)
It's definitely an interesting idea -- but I don't see yet how it could be implemented.
