Those appear to be cutaneous horns from what I can see in the pix. If they are then two of my cats have those. When they get too long I just snip them off as they are a callous like thing and have no blood supply. Check with your vet first before you clip though just in case.

Here's another site about them with several pix. http://www.messybeast.com/horned-paws.htm

They aren't necessarily a Manx thing. I had a full blood Manx as a child and he did not have those, and the two cats I have now that get them are not Manx cats.