What is a "Valid Species"?
"Valid" in this case is just emphasizing that the Heterotermes perfidus on this island are a different (real) species, and not just a variation of their closest relatives. You need not get bogged down with how we determine species, or define species. It just means that this taxon (named group) exists as a separate species: a "valid" species, based on differences that some taxonomist deemed important enough to describe and give a name. Others may disagree, and write a paper to synonomize it (sink it into another existing species), but that is part of the messy business of taxonomy.