LEAD: Boasting azure waters and white sands, a Turkish lake that NASA thinks hides secrets about Mars threatens to become too popular for its own good. Lake Salda gained international renown when US scientists began poking around in preparation for the Perseverance rover mission, which has been beaming back videos from the Red Planet since February. But activists fear that the interest could open the floodgates to tourists, and threaten the very ecosystem that made the lake special in the first place.