Yellowing of foliage can be caused by too much sun exposure, especially in winter. It can also be due to other environmental conditions such as poor soil nutrition, pH, and/or drainage,  so a soil test might help to see if the plant needs food or pH adjustment.

When blooms are brown at the edges that is usually caused by freeze injury, but possibly water issues (if its too wet or too dry plants can't bring in enough water and on warm days the furthest petals/leaves will dry out).

Browning of spots in the middle of the bloom at this time of year is caused by Petal blight which does ruin the blooms and cause them to fall very quickly. You will need to clean them up and burn or remove them from site to reduce the problem in the future - no currently recommended sprays to assist with this

The symptoms of petal blight can be easily mistaken for cold damage, but do not affect every flower on a plant. Flowers are usually affected within 48 hours of being infected.

When a plant is infected, the first sign are small brown speckles on the petals. These spots will ultimately turn the flowers completely brown. Not all flowers will be affected, and it may seem rather random.

When the blooms fall, blight can be confirmed by looking at the bottom of the flower, where a canker will have formed at the point where the flower was attached to the branch.

Spores reproduce in the soil. So, gathering and disposing of fallen blooms is an important factor in controlling the disease. Treatment with fungicides is not recommended. Once all your blooms are collected and destroyed you can mulch the area to bury spores that would infect your blooms next season.

The gray mold could be some powdery mildew (rare) or more likely some sooty mold that would wash off easily with rain. Check the underside of your camellia leaves for scale insects and/or the sap could be falling from a tree above the camellia too. Doesn't look too serious to worry about. 

A soil test might be good though to make sure the pH and nutrition is suited to your camellias and of course shade, and good drainage are also key to reducing stress.