Mango tree has white specks on leaf undersides
I have my pickering mango indoors for the winter and recently noticed white specks on the leaf undersides (see pic). The specks wipe off easily and are definitely not insects. I've had about 5 leaves drop over the last month (not a lot but still concerning), and I wonder if it's related. I previously had some spider mite issues and sprayed with a mild neem oil solution, but I'm pretty sure that the spider mites are gone now.
Do you know if these specks are a problem? Thanks!