The green bar means that this part is cached in the RAM and should play in real time.
Fusion in Resolve can't access all the RAM you have because it needs to reserve more for the rest of Resolve. You could get a little more RAM out if it in Fusion Studio.
But you might want to look at disk caches. Activating this for node branches that are done could lead to better performance in the following nodes.
Right click a node and select Cache to Disk. I'm not too familiar with how Fusion in Resolve handles disk caches but I would assume it's like Fusion Studio and the default location can be changed in the settings.
You preferably want to save the disk caches on a fast drive so I/O isn't a huge bottleneck. Fusion will default to writing Fusion Raw files which can get quite big. You can cache in EXR, too but that's not interactive so you might as well use a Saver if you go that route.