I have a standard powerline10k install, recently updated config (this has been an issue for months though).
In iTerm2, with recommended font, I get a small cyan triangle to the left of every prompt. It persists when the terminal window is cleared and when the prompt moves down. This does not appear in Terminal.app.
I came back to this after living with it for months. It occurred to me to check Terminal.app and, after setting it a profile up there with the same fonts, I confirmed that this issue is specific to iTerm 2.
So these turn out to be "mark indicators" that are part of iTerm's shell integration. I was able to disable the display of these marks by editing the Profile (Preferences -> Profiles -> select relevant profile) and then turning off the display of marks (Terminal -> under "Shell Integration", turn off "Show mark indicators."