Lots of purple stringy algae on my live rock

[OCDP]

So I have had this string like purple algae growing on my rocks.. 2 in particular that are the worst. It's quite annoying as it takes away from the actual live rock appearance.. it just looks like a patch of purple string really.

I was wondering if there is any way to get rid of this stuff... please do not say manual removal as I have tried this many times, it's very very hard to pull off the rocks for one, and it grows back like a weed... is there anything I can add to the tank to eat it, etc...

Thanks