He apparently is mounting the toy, which isn't that unusual for fixed cats.
The procedure mainly prevents procreation and removes the uncontrollable urge to look for mates, while technically the cat is still capable to 'perform the act'.
I've found cat owners tend to be a bit reluctant to talk about the behavior, but if you ask around openly, you'll find it ...
Your cat could be protecting your other cat. He could be keeping an eye on her due to previous experiences or an attack. Either that or he just wants to keep an eye on her for the heck of it. I would say see if it gets worse, then take them both to the vet.