While it doesn't happen very frequently it is possible your tank has cycled, it is not likely, especially with the shrimp still decomposing in there. You might want to test for ammonia with another test kit to verify your test kit.

Personaly, I'd give it some more time.

Purple-Up has been debated for quite some time, general consensus is it's a waste of $$. With the proper levels of calcium, alkalinity, and flow coraline will grow just as quickly without the additive.