This particular strain of protosagania survived by following two simple rules, "strength in numbers" and "defense is the best offense".
In response to the first, they came to grow in clusters, covering areas of the ground beneath whatever body of water they were in. Growing side by side through budding, they can cover relatively large areas given time. They secure themselves into the ground using a single barbed proboscis, resulting in a rather strong grip on the ground and rendering them hard to remove. Occasionally, one newly budded solumcrusta will get caught by a strong current or passing swimmer before securing, and be taken away. Eventually when it does reach the ground, it will root and create a whole new colony. This is how the solumcrusta has become so widespread across Sagan 4. Any water clear enough for them to photosynthesize is a potential home for them.
As for the second half of their survival policy, they have developed a thin shell around their soft innards in order to better protect themselves against predators, like Cnidolium simplistica. Though thick enough to protect against most other microscopic organisms, it can be easily crushed if the creature is big enough. It's shell is comparable to the exoskeleton of many insects.