It could be too much organics from dieing algal cells, but more than likely bacterial contamination. Forgot to mention that I boiled my water in a kettle before adding the salt to slightly sterilize the procedure also known as semi axenic methods. If the algae you used to start the culture has a lot of bacteria in it it could cause more problems, are you storing it in the fridge? I would smell it to check it is ok, open the top and give it a few seconds then have a smell. if it smells slightly of rotten eggs then it is contaminated.