Heh, a funny collision of terms.

The green Merged means that the pull request was submitted into the branch.

The DO NOT MERGE text is an instruction for automerger. Android is developed in a truly startling amount of related git branches. Automerger is the tool that propagates commits back and forth to make sure pull requests get to all flavours, versions and devices.

DO NOT MERGE tells Automerger not to propagate that pull request to newer versions of Android, i.e. it's a fix for the currently released version that's not relevant to the next development version of Android for whatever reason.

Also seen, although more rarely, is DO NOT MERGE ANYWHERE which tells Automerger not to propagate the pull request to any other branch other than where it was originally submitted, including branches for related products on the same version of Android.