Possibly maple scale. I remove them manually but my maples aren't that big. There is probably some spray you can use if you believe in better living through chemistry. Some type of scale seems to occur in limited amount on my maples yearly - but they seem to do limited damage (I do give some of them the shaft)

Look up "scabies" ... it's a parasite that infests hands. And DO NOT shake hands with him.

Looks like a type of Opuntia. Maybe similar to O. schickendantzii. Not sure of the species, normal growth habit, or price though.

Certain types of galls will have an exit hole where that the gall wasp uses, making it look like an insect nest.