my cat is randomly attacking my other cat

My male cat Simon (over a year old) randomly attacks my female Sunflower. They are usually the best of friends :(

We have moved into my parents house while I wait to move in with my boyfriend (I can’t right now due to him going for extensive job training soon). They weren’t always like this where he’ll randomly bat at her and hiss. I understand this has been a stressful time for them being in such unknown territory and the added stress of my mom little dog and male cat (which her dog doesn’t ever bother them unless they’re trying to take her food).

What can I do to help them? Sometimes they still play and cuddle but it honestly hurts my feelings that Simon will do this because he doesn’t even act like this in general. He doesn’t ever hiss at others unless it’s my mom’s cat nowadays.

Cat tax below: