Akey difference between green algae and land-based plants is that         

Algae are a unicellular body found in water while land plants are the multicellular body.

Explanation:

Algae are aquatic organisms with the unicellular body that have the ability to perform photosynthesis. They are not differentiated in leaves, stems, and roots. They found generally underwater.

Land plants are multicellular differentiated into stem, leaves, and roots and connective tissue also found in them.  

Thus, the key difference in algae and land plants is that the algae are unicellular while land plants are multicellular.