Cleaning terrace tiles - any products to get rid of limescale/engrais stains?

We used some engrais (which was of horse origin) on our tomato plants on our terrace last year & when we watered our plants found a rather unpleasant stain spread across the tiles on our terrace. This stain has now turned white & we’re not sure if it’s limescale or the reaction of the engrais to water/weathering.

We tried scrubbing it off with a terrace brush & soapy water but it didn’t budge.

Anyone got any ideas of any products to remove this?