I know that they are different but I am kind of at a loss for how to explain the difference, but I may be overthinking it. In simple terms, my understanding is that a latent mean(intercept) is an estimation of a theoretical construct that is equal to 0. You can rescale the latent mean by fixing the variance to 1 and setting the intercept of one of the indicators to 0. In this case the latent mean will take on the scale of the indicator. An observed composite mean is simply the mean of the indicators of the factor. The main difference between the two is that the composite mean includes the measurement error of the observed variables, while the latent mean adjusts for this.
Am I missing anything else here? Also, I assume that it would be inappropriate to compare the latent mean from one study with a composite mean from another study?