Purple polyps

[Dearth]

So I found one of the rocks I have is growing a colony of purple polyps some people consider these a nuisance coral others enjoy its appealing looks and searching through this and other sites have not found a satisfactory answer so I have a couple of questions.

1) How fast does it spread?(given the amount of variables involved a round park gestimate will suffice)

2) Is there an effective means of controlling it if it reaches plague proportions?(Biological, chemical)

3) Can it smother or damage other corals?