Scale insects. They need removing with a brush and some methylated spirits.
The adults do not move, but the juveniles do. Now is the time to spray against them, if you use insecticides. Find one which kills scale.
The adults are wax covered so use a horticultural soap based insecticide if you don't want to use or cannot get methylated spirits.

Thanks ODB

yes I agree with Owd's diagnosis and remedy.

Looks like the “cottony maple scale” which also infests acers. It will attack the whole tree- trunk, branches and leaves. It will have to be sprayed with insecticide. If it is a large tree it will be very difficult to control the infestation if not impossible sorry to say.

here is the link to the RHS advice page re cushion scale.
https://www.rhs.org.uk/biodiversity/cushion-scale