Yes, this is a type of spindle gall and it's not harmful to the tree and no action needs to be taken. It's caused by eriophyid mites, which have a wide population of plants, and as they feed, it initiates gall formation on the leaves. 

Here's an article from American Nurseryman, which has a photo and some additional information: https://www.amerinursery.com/pest-management/nail-galls-linden/

For general information on galls, see: http://extension.colostate.edu/topic-areas/insects/insect-and-mite-galls-5-577/