Welcome to AA!!
You can just scrub those rocks real real good. And soak them in a bucket of hot water. Use a dish washing brush (works well) get the tough kind.
I've done it plenty of time. Using the brush and to get into the little crevices I used a tooth brush.
Again welcome to the forum
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley