This was a novella contained in a large edited volume of Sci Fi short works. Since I read it in the mid 1960’s to 1971 time period and it had a library binding, it probably dates from early 1960’s.
The plot is as follows:
Humans underwent a diaspora throughout the local galaxy; then at some point colonies lost contact with each other and with Earth and cultures evolved on their own pathways. At the point in time when the story takes place, an interplanetary Human government is trying to locate old Earth colonies to bring them back into the fold. This is apparently a very desirable event for the other cultures, as they get all sorts of economic benefits by being in the human club. Thus, many civilizations of near-human look-alikes also try to get into the human federation (Yes I know, what are the chances? Convergent evolution can only do so much. )

So inspectors investigate new applications to the federation to determine if the people really are descended from ancient earth colonists. The lead character in the story is an inspector/investigator. He is following up on the investigation and mysterious disappearance of an earlier investigator. He has a copy of the previous investigator’s rather cryptic journal, which mentions “Musci” in relation to the people of the planet. Musci? Is he talking about houseflies (Muscidae)?

Turns out, the people look much like humans, but clearly are not; they reproduce by alternation of generation, like mosses and all other land plants (though it’s only really obvious to the naked eye in mosses and ferns). “Aha! Not houseflies, but mosses!” the narrator of the story thinks. (“Muscinae” is an outdated name for the mosses, now called Bryophyta.) There is a diploid generation that gives birth to a batch of haploid babies (plants do it with spores). These babies are spirited away (out of sight of nosy humans), and grow up to be either pure haploid males (one set of chromosomes plus a Y-chromosome) or pure haploid females (one set of chromosomes, one X-Chromosome). The author describes them as very handsome/beautiful, the essence of the ideal male or female. These people have sexual reproduction, give birth to diploid babies, and die. The diploid adults raise the diploid babies (if I remember correctly) and the haploid people raise the haploid offspring of the diploids.

I really would like to locate this work, to use as a side note in teaching introductory biology lectures on plant reproduction and how strikingly different it is from animal reproduction.