What does the small circle symbol mean on the compass/HUD?

Solution 1:

This is a "Custom Waypoint". It is marked by looking/scanning a structure with your Zoom Visor. You probably set it accidentally as I did the first couple of times. To clear the mark, go to the waypoint and scan it. It's a handy way to mark a trading post so you can find it again as you mine and fill up.

Solution 2:

There are two types of white dots that can show up on your HUD.

If the white dots are only visible while holding your scanner for 3+ seconds, then they are locations where fauna will spawn, but are too far to tell if they've already been discovered or not.

If the white dots are always visible and do not pulsate, then they are points of interest that you've scanned - something that can be interacted with in some way, such as an Exo-Suit upgrade.