The icon in the circle under the status indicates an environmental hazard that your suit protection has exhausted itself against.
Your apparel, space suit, helmet, and pack all contribute to a certain amount of resistance to various hazardous conditions. When you leave a sheltered area (your ship, a building via an airlock, etc.), that protection begins to degrade. Eventually that protection wears out completely and you become vulnerable to the hazard. As the protection wears down, your watch will start beeping faster and faster to indicate this.
Once your protection is exhausted, your watch will show the icon for the hazard you're no longer protected against. (There may be multiple hazards in a given location, and your protection against each will degrade at a discrete rate.) Further, your status in the data tab will also show the hazard icon.
Hazards will appear in a circular icon. If you then contract an illness or a condition, it will appear as a triangle.
The hazard in the original post was for an airborne hazard. You'll see this show up on planets where there's a sandstorm going on (eg. Mars.) Alternatively you may see it if you step onto a toxic vent. This hazard usually causes the lung damage condition.
Here is an example with a different environmental hazard: thermal. In the first screenshot, you can see my watch shows a circle with a teardrop in it -- that means my suit's protection against thermal environmental hazards has been depleted. You'll also see a small triangular icon with three droplets in it: this is the frostbite I contracted by spending extended time exploring in the freezing rain after my suit protection was exhausted. The second screenshot shows the data pane with the same icons.
So, to summarize:
- circular icon indicates that you're exposed to a hazard that your suit is not protecting against; the icons match up with the suit/apparel resistances
- triangular icon indicates an illness or condition that you're suffering from
The above is what these symbols mean in general. The fact that I showed this on my status when I was in my ship's cockpit (a safe place) is probably a bug. (It's a Bethesda game, what did you expect?)