Q.White Yellow Dust And Spots On Areca Palm Leaves
Note- These are not mealy bugs for sure I spotted a lots of white yellowish spots underside of the leaves of Areca palm and some are even dusty (wipe off on rubbing) also the stem is having black brown spotting. Please tell the reason and cure.

Certified GKH Gardening Expert
It looks like a fungal infection from, either, previous insect infestation or previous overwatering, or even both! The reading on the meter is dry at that particular moment, but I cannot see how far into the soil it is in, how large the container is compared to the plant, and how much of the soil is not colonized by root, as well as the type of soil that it is in.
There is a bit of information missing from the photos, but from what I see... It is an infection from roots remaining too wet for too long, or from compacted soil, such as clay.
Here are some articles that will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/using-topsoil-and-potting-soil.htm
https://www.gardeningknowhow.com/houseplants/areca-palm/growing-areca-palm-indoors.htm

Sir the pot is 12" standard plastic pot and As you said that clay like soil can effect too, i think that's the reason because the meter is set for Ph value of soil and not moisture and is half way deep. Also the plant has been potted in the same pot since a year and I have no idea of root occupancy,. But on top of that thanks a lot for guiding me, i will surely do something.