The suggested link between fibropapillomatosis (FP) in Hawai'ian marine turtles and non-native macroalgae is indeed very interesting. As mentioned in the Introduction to the present article, FP became a problem for Hawai'ian green turtles only in the 1980s, incidentally when the same problem emerged among marine turtles in Florida. I always wondered how an epidemic could start-off simultaneously at two geographic locations as far apart as Hawai'i and Florida. But then, I stumbled over some statements of Celia M. Smith, Professor of Botany at the University of Hawai’i, who testified on April 15, 2005, regarding the impact of marine invasive algae before the US House of Representatives. Among others, she mentioned that in the 1970s various species of exotic algae were introduced from the Philippines and the Caribbean to Hawai'ian waters. There were both legal and illegal introductions. Most often, the algae were placed in pens on the reef adjacent to the Hawai'ian Institute of Marine Biology in Kaneohe Bay. Citation: "… Water motion in that region was sufficient for parts of the plants to break and be carried off the reef, eventually allowing for the escape of these algae to other regions of Kaneohe Bay. …". Moreover, she mentioned that the red alga Hypnea musciformis, which is specifically mentioned in the present article, readily spread from there to other Hawai'ian islands and posed a particular problem for Maui. (Kaneohe Bay and Maui mentioned in the present article as most highly infested with FP-cases.)
The authors of the article speculate that elevated arginine (Arg) levels, passed to turtles as they forage on the red algae, may be a factor promoting the FP-tumors. They argue that high Arg may help the FP-associated herpesvirus (Chelonid herpesvirus 5, ChHV5) to replicate and somehow overcome the turtle's immune defenses. I do not have arguments against the views of the authors. However, I like to suggest that maybe a vector for transmission of ChHV5 may accidentally have been co-introduced with the exotic algae. My preferred possibility would be the marine leech Ozobranchus, which has been suggested as a potential vector (Greenblatt et al., 2004). Unfortunately, the Ozobranchus species that might be involved was not specified in the publication. However, Ozobranchus branchiatus is known to have a preference for green turtles (Chelonia mydas), which are the main victim of ChHV5, and to inhabit both the Atlantic and the Pacific Ocean (McGowin et al., 2010). Virus-infected O. branchiatus could have been brought from the Caribbean to Hawai'i together with the newly introduced algae. Since the turtles like to forage on those, the leeches would also have met immediately their preferred host. Interestingly, the authors of the present article dated the introduction of the non-native algae back to the 1950s. However, my timing (the 1970s, based on Prof. Smith's testimony), introducing the foreign algae only a few years prior to the onset of the FP-epidemic, makes even more sense and strongly supports the present authors' reasons to think about a correlation between the two events. It would be quite interesting to know, whether or not the Hawai'ian O. branchiatus have direct relatives in the Caribbean.

References
Greenblatt et al., 2004. The Ozobranchus leech is a candidate mechanical vector for the fibropapilloma-associated turtle herpesvirus found latently infecting skin tumors on Hawaiian green turtles (Chelonia mydas). Virology, 321:101-110.
McGowin et al., 2010. Genetic barcoding of marine leeches (Ozobranchus spp.) from Florida sea turtles and their divergence in host specificity. Molecular Ecology Resources, doi: 10.111/j.1755-0998.2010.02946.x.