To get Archelon Algae, you need to tame an Archelon, which will passively produce the algae over time.

Algae can be used for fast and effective taming of herbivore/omnivore creatures. Algae is 4× faster taming than Crops and a much higher taming effectiveness that's pretty close to Kibble.

Taming effectiveness comparison of a level 150 Trike (at 1× taming):

Kibble: 99.8%

Algae: 98.8% 🌿

Crops: 83.3%

Mejoberries: 73.8%

Berries: 55.6%

Uh, so.. People can absolutely steal algae from ur tamed archelons, even if it's PvE, so, do with that what you will