Chroma abilities check only mana costs, which are found in a card’s upper right corner. They don’t count mana symbols that appear in a card’s text box.

Chroma abilities count hybrid mana symbols of the appropriate color. For example, a card with mana cost {3}{U/R}{U/R} has two red mana symbols in its mana cost.

This is a characteristic-defining ability, which means it functions in all zones. You include the symbols in the Stalker’s own mana cost if you need to determine its power and/or toughness while it’s in the graveyard.