Don't look to Sepro for advice or guidance. Those of you who have read my posts over the years will know my opinion on Sepro as a "distributor". I fear that they don't seem very interested in promoting Phoslock as it could impact their sales on algaecides etc.
PET have cleaned up hundreds of algae outbreaks by removing the phosphorus. As REllison suggests, you remove the nutrients the algae does indeed die. No need for algaesides.