That chemical is only a temporary bandaid. You need to fix the underlying issues or it will just come back.
You aren't testing everything you need to test for. Phosphates is one of them. You can remove all the nitrates you want but if you aren't getting rid of phosphates as well then it won't matter because the algae and cyano
will still grow.
Have you cut back on feeding? And cyano
grows in areas of low flow. Try adding more flow to your tank.