Your tree may have a couple of insect problems. The webbing sounds like it may be from fall webworm, which does attack sycamore trees. If the webbing is on a small tree in a location where you can reach the webbing, the easiest control is to physically remove the webbing in the evening when the worms tend to gather back in the nest. Crush and destroy them. If the webbing is on a large tree, the amount of defoliation that the worms do likely will not permanently harm the tree. This late in the growing season, the tree has built up enough energy reserves to come back next year without suffering too much. If you decide that you would like to spray an insecticide, it would likely take a commercial applicator to reach up into the large tree.
The black ""seed-looking things"" on the leaves could possibly be frass from the feeding of the caterpillars, or it might be sycamore lace bugs. Sycamore lace bugs suck plant sap from the leaves, causing the leaves to develop speckling of lighter yellow or green where they feed. If there is no speckling, it may just be the leftovers from the caterpillar feeding that you see.