They’re able to return to polyp stage due to a cell change in the external screen (Exumbrella), which allows them to bypass death. As far as scientists have been able to find out, this change renders the hydrozoa virtually immortal. (zmescience)
To put this all in laymen's terms, this organism is able to revert to it's adolescent form after living it's full mature life... which IMHO is really interesting!!!