Arrow down and right icon in HUD
Solution 1:
Looking at this page, that symbol is for Affinity Spike.
From the Wiki, this seems to appear when:
Kills from melee attacks grant 10% / 15% / 20% / 25% / 35% / 45% more affinity.
- Activates on calling out the Operator. Lasts throughout the duration of the mission.
The appears to be part of the Naramon school. Reading the Focus 2.0 page, you select a Tenno School after the Second Dream quest. I assume this is the school you've selected.