Is it possible to give different icons to a cluster group when it starts unclustering?
The red circles with numbers are clusters, but part of a bigger cluster when zoomed out.
Is it possible to change the marker/icon of the marker cluster in this present zoom depending on the markers that they have inside them? This without clicking them or anything, just change depending on the values of the markers that are hidden? I have tried accessing the layer that is clustering all of them but I haven't been able to find the markers themselves, much less know how it would be possible to change the appearance of one without changing the other.
It looks like you already know how to customize your clusters appearance.
In the case your question refers only to the number of contained markers, you would just need to use a slightly modified version of the default iconCreateFunction to adjust the threshold values. When markers are removed/added from the MarkerClusterGroups and a cluster is de-/populated in consequence, its icon is re-drawn automatically. Customising the Clustered Markers
Now if you want this appearance to depend on some data from the contained markers, you would simply need to use cluster.getAllChildMarkers(); within your iconCreateFunction to get the array of markers contained within the cluster being styled. Then iterate through that array of markers, look for your data and create an icon accordingly.
Then, I understand that some data attached to your markers is changing, without any user action, and you want your clusters to update their appearance in consequence? In that case, please refer to this related question to upgrade your markercluster plugin with the new refreshClusters() method.
If your iconCreateFunction is properly designed, you do not have to worry about which clusters are changed. In fact, ALL clusters can be re-drawned, but if their markers data has not changed, they will get the same icon.