It could be mold - powdery mildew. Bathrooms are humid, and succulents are arid climate plants. Even if you have other plants not having same problem, it could be just question of time. There are other plants that would enjoy more humidity, but not succulents. Besides that, it is usually darker in bthrooms and succulents need lots of light. Looking at your photos, they are showing some etiolation. Leaves are growing too far apart because plants are not getting enough light.

Remove the dead and dying leaves. Overall it looks fine unless you can post photos of the new leaves showing a major problem. Was the residue there when you bought it? I see a lot of plants from Florida (Home Depot and the like) where the plants were treated before shipping. Usually a wet sponge/paper towel can remove most of the residue. Leaving it in a shower without enough light is not good for it. It was enough shock for it being shipped hundreds of miles to live in a home when it was nice and happy growing in a commercial greenhouse.

"the last 2 years they’ve started to open the tips of their buds months before they should be blooming." - It is completely normal for all types of camellias to produce flower buds months well ahead of the time when the flower buds actually open. Each cultivar's flower bud will develop from late summer to sometime in the fall and open at a different/appropriate time for that specific cultivar. Camellia sasanqua flower buds will typically develop and open flower buds in the summer/fall months, whereas Camellia japonica flower buds will develop flower buds in the fall and these will open later, anywhere from late fall through spring (April-May) depending on the cultivar, your geographic location as well the current local winter conditions. Flower buds can be damaged by cold temperatures. The flower buds can either fall/drop to the ground or turn completely or partially brown. Flower bud drop can -for example- occur when there is an environmental shock such as a large temperature drop from the 80s to below freezing in less than a day. If the flower buds turn completely brown due to cold damage, they are dead and will either fall quickly to the ground or not. Partially browned out flower buds may or may not open; it is difficult to guess what will happen with partially browned buds as it depends on the extent of the injury. To protect the plants and minimize the chances of flower bud problems, I maintain 3-4" of organic mulch all year around the plants, stop fertilizing 3 months before my average date of first frost and I also water deeply before temperature drops well below freezing. Small, quick temperature drops to freezing or slightly below, followed by warmer temperatures in the morning hours normally do not cause problems. The camellia plant usually survives occasional temperatures as low as +10° to 0°F. Camellia flower buds are damaged at temperatures below +10° to +15°F. Dry soil conditions promote flower bud injury. Open camellia blossoms normally are damaged at temperatures below +26° to +28°F. If your local winter temperatures are such that flower bud damage occurs often in most years, that indicates to me that the cultivar has a blooming time that is not appropriate for your location and one should consider using a cultivar that blooms earlier (before the flower buds are winter damaged in your area). Camellia sasanquas are the earliest blooming camellias, starting somewhere in the fall (varies by cultivar). Camellia japonicas and hybrids can start to bloom anywhere from December to April. If damage to japonicas tends to occur in March, consider using cultivars that open blooms before March (some of the earliest blooming ones open flowers in December). Camellias that are blooming may not have flower bud issues but may also temporarily delay opening more blooms for a few weeks (or even as long as a few months) if climatic conditions suddenly get very cold. From your pictures, I noticed that the soil appears to be dry and does not have any or much organic mulch. That may dehydrate the plant in winter or when drying winter winds occur. You may want to see if maintaining the soil moist at all times as best as you can and mulch help in future winters. I am assuming that the soil has not frozen where you live. Note: the first year that you got them, the plants were forced to bloom early by the wholesaler so they would arrive blooming at your local plant nursery. That is what wholesalers of all plants often do, not just camellia wholesalers.

@SylviaWW 9a Hot dry SoCal in hot conditions where the sun is brutal, any distance you can put between the physical plant tissues and the hot, reflective, heat-radiating wall surfaces will help. Allowing air circulation behind the roses, between them and the wall will not only help to permit the heat to dissipate, but will also reduce the amount of water stress caused by the hot wall literally cooking the moisture out of the roses. The hotter it is, the more they sweat (transpire) and you can literally force them to contract fungal issues, most notably rust and mildew, from severely water stressing them. Growing grass around them will increase the possibilities of water stress as the turf competes with them for water. The rose feeder roots will grow within the top few inches of soil and that's where the grass roots are. Ideally, there wouldn't have to be lawn around them, only a deep organic mulch to keep the soil moist and cool. But, the reality is, you have to do, what you have to do. Good luck!