Yes they are! Mustard algae is hard to eradicate. You really need a good test kit so you can test the high chlorine levels which will be needed to kill it. The kits you can buy locally usually only read chlorine levels up to 5ppm which isn't gonna do the job. The kit we recommend is the Taylor K2006 or 2006C which you can pick up through the test kit link in my signature below.
Also, please put your pool's information into our pool chart as that makes it easier for us to give you good advice.
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