Unless you go woof at the postman, and drink water using your tongue, hibiscus is completely safe, there are some suggestions that the plant (not the flower) is dangerous to dogs, but even this is anecdotal, it is completely safe for human consumption, the hibiscus flowers add a beautiful red color to the tea and tastes like fresh sweet rasberries, with a slight bitterness to it.

It really has a great falvour, and even just on it's own makes a really nice tisane

Techincally speaking any tea made from anything other than the tea plant (Camellia sinensis) is not tea, green tea, white tea, black tea, oolong tea etc are all made from the exact same plant but grown using different methods, and processed in different ways, black tea for example is fermented whilst green tea is not, and white tea is picked from very young leaves and only slightly fermented.

Any "tea" made from herbs, sticks, bark, or a blend of herbs etc is called a Tisane.