When you say you have had no problems with this skimmer, what are you comparing it to?
Most aquarists use skimming as one of the cornerstones of their systems. There is no doubt in my mind that your skimmer does create air bubbles and does remove some DOC, but once you really start stocking that system, I very much doubt whether it will be sufficient.
I would recommend opening up your aquascaping, as the rocks lying flat on the sand only serve to trap nutrients (algae fuel) which will lead to problems later on.
The flow dynamics need some consideration, as it seems not to be lifting detritus from the bottom of the tank up and over your overflow... vital to get it processed properly.
That cauliflower coral looks like a coral from the Nephtheidae family, a non photosynthtic species, in other words, it requires a lot of feeding and a lot of flow. Not saying it cant be kept, rather it shouldnt be kept by inexperienced reefkeepers.
What treatment are you doing for white spot?
Do you have an auto top up in the sump?