Ok so I did a bleach soak for 24hr then power washed the rocks an they turned out pretty white. Did a 1.5hr soak in muriatic acid and power washed it again. Rocks turned out dirtier than after I bleached it. I did notice some loss of rock mass. About 10% or less I would guess. I have it in a bin with ro water and a hang on back carbon filter. Would it turn white again after a few days ro soak?

They should come out a bit whiter, but after eating away the surface of the rock, all the residual crap made them a little darker. I personally wouldnt worry about the fact that they arent really white. They (hopefully) will be covered in coraline after a good cycle anyways. If you like the white color, you could do another bleach bath. But afterwards I would do a long RO water soak with prime, and then sun dry for a week or so to make sure the bleach is all gone.

What's the need for prime when I'm using or water?

If you decided to re-bleach the rocks for color, you should always add a de-chlorinator. But if you are happy with the color, and are done with the acid bath, then you are good to go without the prime.

They'll turn white again if left in the sun

Yeah mine seemed to do that for sure