To start with, while Acem meditation is the name of one specific technique, mindfulness covers a range of different techniques. Some of them are quite similar to Acem meditation, while others are very different.
Second, while the meditation object in mindfulness techniques is the breath, it is a sound in Acem meditation.
Third, there are differences in how you use your attention. In Acem meditation and some mindfulness techniques, the attention is non-directive, with a free mental attitude. In other mindfulness techniques, the attention is focused and concentrated.
Fourth, there are differences in how you deal with the spontaneous activity once you notice that your attention is not on the meditation sound anymore. In Acem meditation and some mindfulness techniques, you just go back to the meditation object, while in other mindfulness techniques, you are to get finished with your thoughts first, decide to let them go.