Red slime is usually a consequence of using tap water, overfeeding, high nitrates/phosphates, and low flow. It sounds like tap water and low flow are not your issues.
Your phosphates are reading "trace" because the red slime is using them all up and they are not staying in the water column long enough to read on a test kit. For this reason, phosphate tests are really inaccurate. You can be reading 0, but have a huge phosphate problem.
The crushed coral will not help. It's not good stuff. Way worse than sand. The particles are large enough to trap old food and detritus, which then rots away. Your CUC can't get to it like they can in sand, so it just pollutes the water.
Do you have a CUC? Like nassarius snails for keeping the substrate clean? If you had nassarius snails/sandsifting goby/sandsifting stars, then the sand was not your problem before, as those guys were keeping it clean.
Another idea is the live rock you added. Where did you get this rock from? Someone else's tank? Live rock that has been in a tank with high nitrates and phsophates can absorb them and then leach them out slowly over time, leading to a neverending algae problem.
I wonder if you removed that new rock that you got, if the slime problem would go away?