They may even be reproducing sexually when they start appearing at the other end of the aquarium. They do so in mine, every (other) month they get pinkish colored eggs that are released in the water column.
I have them growing all over the live rock as well, but can't be bothered as they don't have any stinging powers. All other corals (mainly sps and lps) I have will kill them once they touch so the other corals simply create their own growing space if necessary. They also have an advantage as well as they are great in polishing your water, I always have zero nitrate and phosphate. Personally I also prefer the color of these than that of bare live rock.
A paste of calcium-hydroxide will kill them permanently if you want to get rid of them.