If you're looking for an alpha mosa (for 100 black pearls), kill those regular ones if you find them. There's a fair chance an alpha mosa will spawn.

I spent 2 days circling around the island but no alpha mosa. I decided to kill those regular ones while on my way back to base, and there she is an alpha mosa right in front of my base.

I used itchy to kill it. My itchy was 300 health, while the mosa was 54k. It took me 1 ark day.

To TUSO: I'm coming for you bae😍