Hi Chiara,
To be honest, I'm not aware of any community accepted protocol for citing a forum page. Perhaps others will chime in. In the meantime, are you looking to cite the algorithms for MODIS and SeaWiFS and their differences? The information in the forum post you list is outdated as of the
2009-2010 Reprocessings. The best way to cite the suite of OC algorithms is
O'Reilly et al. 1998. The current, operational description of SeaWiFS and MODIS OC algorithms can be referenced
using this link. The data processing software (l2gen) can be referenced
via this link. Perhaps not the information you seek, but FYI.
Best,
Jeremy