Something may have happened, unbeknownst to you, that frightened, frustrated or startled Mufasa and he is exhibiting 'redirected aggression' toward both the humans in the household and his furry companion. Here is more information about redirected aggression:
First, I would recommend a visit to the vet, to make sure this sudden aggression is not stemming from an underlying medical issue and Mufasa is in pain. Make sure he's urinating normally and there is no problem with him being 'blocked' in his urethra/bladder, as that is a medical emergency in male cats. Hopefully, all will be well. If this is a behavioral-only issue, the following over the counter calming remedies might help:
Note: don't use the oral products at the same time; just use one in conjunction with the Feliway.
If there is no improvement after a few days, then I think it would be time to consider a calming prescription medication like Clomicalm, after discussing the many medicine options with your vet.
There are a variety of these types of anti-anxiety medications and your vet can guide you to the one that would be most beneficial.
You might also consider a thundershirt for your kitten; this makes them feel more secure and less anxious:
I hope things improve, soon with little Mufasa!
Please be so kind as to rate my answer with positive feedback, as that is the only way I receive recognition from the website for my assistance. Thank you very much!
Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply' for clarification or additional information, if needed.