It's good old limescale. Absolutely harmless, but unaesthetic.
The easiest is to just pour acid on it which will dissolve it. Vinegar (aka dilute acetic acid) should be safe for your septic tank, it is also safe to use, so pick the cheapest no-name white vinegar available, you'll need about 2 liters. It works faster if the vinegar is hot, just use a cooking ...
I know this is an old post but it seems to have a lot of recent views... so I hope by adding my experience it will help someone.
I use a heavy-duty (freezer or thicker) gallon ziploc bag and stuff a couple of wet microfiber kitchen towels or terry washcloths inside it, favoring one bottom corner to make a stuffed pointed bag. Then tie it shut with a string ...
You need to find yourself a "Repair Flange" or "Flange Repair Ring".:
Images from Lowes
It fits over your existing broken flange and is fixed to the floor by using as many of the mounting holes as you can - preferably at least 4 IMO.