You cannot store NSW for 2 or 3 weeks.
First, there is no water movement, limited oxygen exchange. Most likely kept in a closed up drum. There are living bacteria inside, plus some algae and nutrients. Without light, the algae dies. The bacteria got short term enough food, until they run out. The water would seems to be OK, but is it really?
The biofilm developing on the insides of the drum, does indicate a cycle process starting. New bacteria is colonizing and growing on the sides of the drum. How far into the cycle you do not know, unless you test ammonia, PO4, Ni2 and Ni3.
To start off a new tank with NSW is "OK". Provided you did start from a unpolluted source. Cape Town waters are a bit high in nutrients but that would be sorted during the cycle process. But to use NSW for water changes, after you left it in some drum unattended for 2 or 3 weeks are risky. Rather use it as soon as possible. How do you preserve ALL forms of life in that drum, without providing circulation, light or food?
Between East London and Cape Town is a lot of difference. East London you still have the Indian Ocean sweeping down. Not as cold waters as Cape Town either. Sure, some microscopic life will be the same, but there will be a lot that differs. Cape Town got nutrient rich waters washing up from the south.
Yes. You can use NSW. But do not store it.
Storing synthetic salt is OK, unless its a white drum and sunlight do fall on it or in a brightly lid area. Algae will eventually form inside it.