Rosehips and oranges are “acidic”. Your sieve/strainer is being dissolved by the acids.

Your strainer is most likely aluminum and reacted with acid in the food

Is it okay to eat ?

I wouldn't trust it. When in doubt, throw it out. Better safe than sorry.

Of course it's OK to eat.....and if it shows any rust...it's not aluminum but iron or steel which is the same element. Taste the fruit and see if it has any off taste before you waste time on something that may taste "tinny" or have an off flavor
Many or our ancestors lived well into their 80s and 90's eating food cooked in aluminum and iron.

I would not have or use a rusted appliance or device.

Rust is iron oxide....good for red blood. Won't hurt you...might harbor bacteria but that is killed by the high heat of cooking. BUT it may give a metallic taste.