Desirable algae - as opposed to unwanted and horrible hair and slime algae is very difficult to keep. To date I've not been able to get it right although I hope I've satisfied the criteria required with my latest efforts.
Previously whenever I've had a great rock with a nice clump of wrack, after a few days its started to fray and darken. Within 10 days its gone and the skimmer has been working overtime. Initially I thought this was a lighting issue so boosted the lights as much as possible. Then I thought it was a temperature issue so brought a chiller. Then I thought it was a salinity issue (see seperate post on salinity). I believe that now I've got all the paremeters right and yesterday added a small amount of horn wrack. Hopefully I've finally cracked it!