Hi,  Yes you identified this insect correctly, it is cottony maple scale and there is an uptick in the number we are seeing this year.  Usually we see a little of this but not as much as we are seeing this season. Aspen can be a host plant for it along with several other ornamentals.  Here is some additional information on the insect  https://pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/maple-acer-cottony-maple-scale    
Here is some additional information on management:  https://pnwhandbooks.org/insect/hort/landscape/common/landscape-scale-insect      If you just have a little you can scrape it off.  If you have too much you can try spraying with horticultural oil or neem oil. A small amount does not kill the plant, but may affect individual branches.  Also, these insects will often attack plants under environmental stress.   Do they trees see healthy otherwise, drought stress can be one factor that leads to this insect, is that a possibililty?  Thanks,  Amy Jo