REQ Parasite info

[John & Su]

Hello
Anyone can give me information about that kind of grey crusts that are glued
on one of my cacti, I scratch t but it come back few days latter.
What is it ? is it a kind of  insect ? (it doesn't seems to move but appear
suddenly)
How to remove it definitively ?
Is it a (easy to find *) product that can remove it ?
How to avoid it come again on this plant (or others) ?

Any info welcome and thanks,
John.

(*) because lots of think are hard to find here in Thailand.)