Hi,

Thank you for your questions. The insects on your Japanese Maple are aphids. There are many species of aphids, from the the abdominal cross-bands it looks like they may be common periphyllus aphids but is is hard to tell from the photo. Aphids produce a sugary liquid waste called honeydew that is created when an aphid pierces the plant’s phloem ducts with its mouthparts, creating enough pressure that the sap passes straight through the aphid’s body and escapes as excrement. This is what you are seeing on the leaves. From the photos your tree looks otherwise healthy. To address the aphids blast them with a garden hose using plain water until they are all off of the tree. The blast will dislodge them and cause them to fall to the ground and, water logged, they will not be able to journey back up the tree and will die.  Keep an eye out and repeat if you see any new infestation. 

Here is a a link to further information on Aphids and their control from the University of Maryland Extension.  I hope this is helpful. Please let us know if you have any further questions. 

Best,

Christine