If your calcium and/or carbonate hardness are low, the water will dissolve the rock to replenish the desired levels of Ca
. The real issue here can be the other nutrients that are dissolved and set free by the aggressive water. These can include, but aren't limited to, phosphates and minerals that will encourage algae.
If you use a turkey baster to blow around the areas affected (yes, I know your powerheads are there), you should see a sediment cloud. If so, this is indicative of low Ca
. Monitor and dose frequently and accordingly.