If he's a rock person then he must actually be pretty old, considering Woo was 70-ish and looked like a human on their 20s. It's either that or he just pretends to be old somehow
Or maybe he's Yashuo's ex and he can shapeshift or something
This chapter has shown many mysteries, Tooru and him showing up everywhere, Tsurugi being a serial killer, a stand attacking Josuke and toss up of the user for it being Tooru or the head doctor. Oh and did I mention, THE HEAD DOCTOR IS 89