The Gemara (Nazir 65b) says that Rabbi Yehoshua learns from the passuk of לטהרו או לטמאו that the case of ספק שיער לבן קדם ספק בהרת קדמה the halacha is טהור. The question is why he needs a passuk to say this if the Gemara (Kesubos 75b) says that the reasoning of Rebbe Yeoshua is because he holds that we go after חזקת הגוף?
One clarification: R' Yehoshua is saying that all of the tanna kamma's cases are tahor, not just the safek case.
That should clear things up:
The pasuk comes to tell us the lesser chidush unless otherwise indicated.
If R' Yehoshua did not hold of chezkas haguf, then a case of white hair before baheres would be a safek and the chidush of the pasuk is that it is tahor, but baheres b/f white hair would remain a safek. Since R' Yehoshua holds of chezkas haguf, white hair b/f baheres is tahor without the pasuk. So the chidush of the pasuk is that baheres b/f white hair is also tahor.
(see Tos' iy hachi who uses the above structure when applying R' Yehudah/Rav to nizkak l'tumah above)