It is hard to tell based on the picture attached, it would be interesting to know more details about your culture.
I would recommend to take a closer look and search for contamination in your culture. Ciliated protozoa or other predatory microorganisms (e.g. Vampirovibrio chlorellavorus) can destroy a Chlorella culture rapidly. Basically, if you have many cells that are destroyed like this, you might want to eliminate this possibility. If you have only a few cells like this, the reason can be natural cell death.
Thank you for your response. We already know what happened. We wanted to test the method of photobioreactor sterilizing with sodium hypochlorite solution and that was the result.