This question is in relation to speculative biology/evolution: A genre of science fiction and hypothetical branch of science which seeks to explore "what if" questions about life using established principles of biology, ecology, evolution and related scientific fields.
An in-depth explanation for the proposed planimal life cycle, and comparisons to life on Earth:
Depiction of ancestral planimal life cycle
The ancestral condition for Earth animals is to have a motile juvenile stage called the larva and a sessile adult stage, and while not all animals' adult stages are sessile, all Earth animals begin life motile. By contrast, the ancestral life cycle of these planimals has them begin life as a sessile, photoautotrophic juvenile stage which I shall call a polyp, which matures into a motile, heterotrophic adult stage. This life cycle may be likened to Earth cnidarians, however even cnidarians begin life in a motile larval stage called a planula, while these planimals begin life sessile. Early stem-planimals and even a few planimal clades do not have a motile adult stage, and are effectively just "plants," which to my knowledge is unlike anything seen on Earth.
Proposed hypothetical evolution of planimals:
1). A heterotrophic eukaryotoid microbe consumes and undergoes endosymbiosis with a cyanobacteria-like photoautotrophic microbe.
2). The now-photoautotrophic eukaryotoid evolves multicellularity, and attaches to the benthos of the photic zone using a holdfast.
3). In order to increase reproductive success the organism develops a strobila much like many cnidarians on Earth. The segments of the strobila, called proglottids, break off of the main body once mature and become planktonic; acting as vehicles for the genes of the whole organism to allow for a faster propagation of gametes; protecting the gametes from predation until coming into contact with another proglottid and exchanging gametes. Proglottids may be capable of some movement via flagella/cilia, however do so rarely.
4). In order to further increase reproductive success the proglottids develop a highly motile, more animaloid mode of existence in order to more easily reach other proglottids to mate with. As a consequence the proglottids become primarily heterotrophic to fuel their now highly active lives, although they may still supplement their diet with some level of autotrophy. By this point the organism has become a planimal.
As far as I know, there exists no organism on Earth which has evolved in such a manner as this. Could such lifeforms exist on an alien world, or are they not naturalistically possible?