It is often a good idea to put dry rock in salt water and a power head, let sit a for awhile.... week or several weeks and then test for phosphates. If you don’t have phosphates, then the rock is probably not leaching them.
You can also cycle the rock before adding it to your tank. Just add some ammonia. You can also add some bacteria culture to speed thing up but it is not necessary.
Your rock looks pretty clean. Normally, I clean dry rock with bleach but that rock looks like bleaching is not necessary. If I suspect phosphates, I will also acid wash rock. To acid wash, put rocks in tap water and add some muriatic acid. The acid will eat through the outer layer of rock. Don’t go over board with the acid because it does destroy some of your rock. And acid wash in a well ventilated area like outside and don’t breathe the fumes. When I handled that stuff in the lab, I used a fume hood. One of those would be handy.
