This is a female whitespotted pygmy filefish (Rudarius ercodes). She is tending to eggs that she has affixed to seagrass (Zostera marina):
This is not an unusual species in Japan. This is not unusual behaviour.
What is unusual is the location.
According to Fishbase, the documented range for Rudarius ercodes comprises "Northwest Pacific: Taiwan, Japan (central Honshu to southern Kyushu) and southern Korea," i.e., temperate to warm waters, not the cold waters of Hokkaido where I photographed this fish. The ocean temperature at the time was something around 8ºC.
To put this sighting in context, I should note that fishes and other marine life get swept north from warmer regions every year by the Kuroshio current. For example, I have seen juvenile yellow boxfish (Ostracion cubicum)—a decidedly warmwater species—in these same waters. So while stumbling upon a non-local resident like this from time-to-time might be unexpected, it shouldn't be too big of a surprise. Castaways from lower latitudes usually do not last long though. They perish once temperatures drop below tolerable limits.
But this encounter gave me reason to think.
By virtue of the fact that this sweet little female was tending to eggs, we know that there was at least one male in the area as well. We also know that both fish were healthy enough to spawn. The eggs were well-developed, meaning that they were viable. Not duds.
In recent years, friends and I have come across other species in locations further north than they had ever been documented. In some cases, we watched the fish spawn, and then found juveniles the following year, along with more adults and more spawning. That is colonisation, an expansion of range.
Shifts like this happen. It is the way of the world.
Shifts like this happening in front of my eyes in Internet time though, is probably not so common.
Though I can only speak to my personal observations and those of knowledgeable friends in Japan, I suspect that other people who are paying attention in temperate and cold waters elsewhere have had similar experiences.
The filefish's eggs were gone when I was able to check a couple of days later. While I did not see hatching take place, I believe the filefish's eggs matured and babies ventured forth.
Whether they survived or not is the question. Odds are that the water would have shortly become too cold for the hatchlings to hang on. The adults as well, for that matter.
But if there is one thing I've learned over the years, it is not to bet against the resourcefulness and adaptability of life.