It's probably "honeydew drip" from aphids. Aphids secret the honeydew, which is harvested by certain kinds of ants that help protect and "farm" the aphids.

Hi, will these aphids kill the tree and will the dripping stop so we can use our deck? Also I notice hard brown things on underside of samall branches. Wonder if this could be scale? Thanks for any help. Jess

You probably ought to prune off a branch and take it to a local nursery or extension agent to see if they can identify the pest. If it's scale, that can be hard to get rid of. They might be able to recommend a spray, but you need to be careful using pesticides around your deck. You don't want to use something extremely toxic where you are sitting and perhaps eating.

Oh, eating toxic pesticide is fun.

Hi, thank you for your help. It is scale. A problem here with the maples according to my local extension service.