Are you sure it's the terracotta that stained it, or could it have been the fertiliser?
I've had fertiliser colour patio concrete slabs and the colour they've been stained is similar to terracotta (in big messy splodges) while the actually fertiliser was white. It's actually some chemical reaction that causes the colour as I understand it.
Unfortunately that doesn't really help, I've not found a way to get rid of it. In my case it's cheap concrete slabs we've been thinking of replacing for the last 10 years so we've ignored it. In your case maybe putting pots back there to cover it is the only way