Thanks reefpod,
It does seem to meet the characteristics specified in the link you provided, since it's growing from 13-week-old Fiji live rock, has tough leaves, and appears to be forming air bladders (the little yellow balls among the leaves).
I had, originally discounted sargassum, as I though that algae had a more matted appearance. This one is growing from a long single stalk that's, currently, about 5" long.