I found these little black "seeds" hanging by silk under my fig leaf. Any idea what this is?

No idea but I'll bet you'll be sorry if you leave them on.  The only positive thing it could be is a beneficial insect or spider eggs but it seems more likely to be a harmful one based on murphy's law.

Any on the top leaf surface?   

Yes some on top but much less.

I agree with Bob--I wouldn't risk leaving those things on the tree. On the other hand, I am curious to see what they are. You should put that leaf in a jar and take a photo of whatever hatches.

Maybe byproducts from an insect of some kind? Check with a flashlight abo0ut 10pm.

Looks like, those are Lacewing eggs that have already hatched.

I am with nkesh099. they look like egg cases, and if they are lacewing, you should be happy.