My first set of Marco rocks were full of gunk.
Turned the water a cloudy white/light grey, even after several rinses.
I ended up power washing them, then soaking for a couple of weeks.
Then they still slowly released phosphates for months.
My second set, years later, was WAY better.
It was clear they had been soaking in salt water for a while. Even had a bit of life on a couple.
No problems at all with those.
So, depending on the batch, those rocks might be the issue.