Rectangulogania
Building off of the foundation its ancestor laid, the Rectangulogania has finally managed to develop a true cell wall, which it uses to help keep its shape as well as form large, permanent mats on the surfaces of beaches all across the southern oceans of the planet.
With help from water-storing vacuoles, these mats can actually survive outside of the water along the shoreline for their whole life, receiving moisture from the surf and sometimes being brought back into the water from it. The consistency is similar to a slime mold, and they have begun to colonize the temperate waters as they fill a purely photosynthetic role and tend to clump into larger clouds when close to shore, so don't compete with other Photosagnia (Kingdom) all that much.
One event which has so far proven unsuccessful is the attempt by the cells to migrate into fresh water around the mouth of the Seal Temperate River. They can so far survive in brackish water, but their full migration may take some time.