I think this is a matter of etiquette and your personal standards rather than California Grill.
They do have an official “Dress Code” on their website, but I’ve seen people show up looking pretty rough straight from the parks. I think California Grill has to be more lenient than a similarly classed stand alone restaurant in the world. Things would have to be pretty extreme before they’d deny anyone admission.
I suppose you are good as long as they are definitively sandals and can’t be interpreted as flip-flops.
“Guests are asked to adhere to the minimum dress code requirements at this Signature Dining experience. Dressier attire is also welcome. Please explore the minimum dress code requirements:
Men must wear khakis, slacks or dress shorts and collared shirts. Jeans may be worn if in good condition. Sport coats are optional.
Women must wear Capri pants, skirts, dresses or dress shorts. Jeans may be worn if in good condition.
Not permitted are tank tops, flip-flops, swimsuits, swimsuit cover-ups, hats for gentlemen, cut-offs, torn clothing and t-shirts with offensive language and/or graphics.”
https://disneyworld.disney.go.com/en_CA/dining/contemporary-resort/california-grill/