Does anyone know the best way to treat multiple plants at once? Sadly what my orchid had has transferred to all my plants my entire greenhouse... I know I have black spots on my leaves plus photos below Thank you so much!

Welcome to the Garden Forum,
Here's a link to information about mealybugs. I use isopropyl alcohol and a q-tip, and/or spray the plant with alcohol. Others report success with neem oil. Treating a lot of plants takes time. You never completely get rid of them, but once they're under control you know to get after them as soon as you see even one. Also treat pot edges and nearby surfaces.
http://ipm.ucanr.edu/PMG/PESTN...
Mealybugs secrete honeydew, which black mold can grow on. Get rid of the bugs and hose off the honeydew, that should help with the mold. If it remains a problem, try using a fungicide.
Always read and follow label directions!

All of the little beasties are difficult to control because they breed so darn quickly. Whatever treatment you decide upon is only going to be as successful as you are in applying it. The more spots that you miss in treatment, the quicker that they come back.

When you want to treat lots of plants you need to spray. Treating plants one at a time will not solve your problem because it takes too long. Personally I have been growing orchids for over 4 decades and I hate to use any strong insecticide or chemical. My method of attack has evolved into using Neem Oil spray that I mix in a gallon pump sprayer. Fill with water, add a tsp. of concentrated Neem and a few drops of mild dish soap. Spray and repeat. Make sure you spray real early in the day or at sunset. Strong sunlight will damage the plants by heating up the residual spray. Neem Oil is a systemic and will be absorbed by your plants. That alone is the best point. Plus Neem oil is anti fungal and a good bactericide.
You can use Bayer's 3 in 1 spray, or a readily available Neem Oil spray from a garden Center. But you are dealing with insects that have a 2-3 week life cycle. Repeated spraying is needed in the beginning. If you spray 90% of the plants, you'll kill 90% of the bugs. The survivors will breed rapidly!!
I grow 200 orchids under lights indoors and the Neem oil is working just fine.