I like using MNNG. It is a chemical mutagen but you can inactivate the chemical by tossing used materials in an acid bath for easy clean up. We have used it successfully for green algae as well as cyanobacteria. I have seen publications using it for a lot of other algae. Need to do a kill curve using the chemical to get >95% kill then treat your Tetraselmis to the 95% kill on a selective medium (preferably) and pull mutant colonies - obviously at 95% kill some will be resistant wild-type but that is why a selective medium would be best.

Be careful since it is a mutagen - gloves and properly dispose of all materials. Acid treat all equipment and reusable instruments and glassware.