Despite the idea that once circulated claiming spirulina was in some way toxic (which turned out to be false), it is very much safe to take everyday in supplement form.
In fact, it goes beyond being just safe. The Dietary Supplements Information Expert Committee gave spirulina a class A rating back in 2011.
That being said, to ensure that the spirulina supplement you are taking is 100% safe and free from harmful chemicals, vet the manufacturer and check to see if what goes into the product is natural.