Oh dear, it's not good that your beetle larva has mites. You need to throw away the soil and get some fresh soil and sterilise it (either by boiling in pan for 30 minutes or put it in the oven at high temperature for 30 minutes). Then flick off the mites with a fine paintbrush and put the larva back in the soil. By the way, larvae is the plural of larva, so as you only have one, you have a beetle larva.