Ethyl esters are made with concentrated fish oils, making it possible to achieve higher concentrations of DHA / EPA.

However the algae oil is a natural product so the omega-3 fatty acids are present in trychliceride form. 

Visual material can be seen via the link below:
https://www.khanacademy.org/science/biology/macromolecules/lipids/v/molecular-structure-of-triglycerides-fats