From the factory, the Dex-Cool systems come with a bit of stop-leak already in the radiator. That is most likely what you are seeing under your cap. I wouldn't worry too much about it, if you coolant and oil look fine otherwise. Just clean off the cap, and see how long it takes for it to come back. You could also probably take out your coolant reservoir and clean it out, too, if it has the rust colored build up in it.
As for the green stuff vs Dex-Cool, as long as all the Dex-Cool was entirely flushed out of the system before the green stuff was introduced, you will be fine. The green stuff will just need a little more maintenance than the Dex-Cool should.