@Sadie - that's ok! The difference is how they are made. Both are just mix with water and drink.
With GHK-
Instant is made by mixing fresh root with water, then straining, and dehydrating the water away. It is every tasty and smooth.
Micro is dried kava, with makas removed, then ground very small. This dissolved with water, but leaves more particulate. The possible sandy mix you mentioned.