If you want to use agar powder in this recipe it is usually 3 times stronger than the flakes, so you would need 1 1/3 teaspooons in place of 4 teaspoons flakes.
Varieties include nori (what you'll usually see wrapped around your California roll), wakame (often seen in miso soup) and agar - agar (usually found in the form of a gelatine like substance).