I'd say you are right in assuming that Ace can smell your other mouse. I would do your best to wash your hands after handling the new guy, and especially before handing Ace, as rodents have a very strong sense of smell, and he can not only smell the new mouse on you, but anything else you've handled (that biscuit from breakfast 7 hours ago, even). You can keep a bottle of unscented hand sanitizer near his cage, for convenience purposes, until you get used to the pattern of constantly washing your hands. It's important to rid them of not only food smells, but lotions and scented soaps, or a co-workers scent whose hand you shook earlier in the day (that may smell like a stranger aka threat to little Ace).
When you treat him, try to only treat him in his food bowl, instead of during handling times, so he gets in the habit, that handling is NOT associated with munching, and that treats are given IN cage, not by during play time. You can take bowl out, add treat, and place back- so he doesn't associate fingers as treat holders.
Also, did you get Ace as a baby? If he WAS a baby, and is now roughly 6 months of age, he has almost finished developing all of his male adult hormones( if not all), which may also be making him slightly aggressive, possessive or cage aggressive. You can help rid cage aggression and possessiveness by eliminating giving him irresistible treats (that he may be hiding somewhere in his cage for later),or by changing things up in his habitat . You can do so, by moving his cage accessories around, one at a time (too much change can lead to stress and shock). Next time you clean his cage, after you've removed dirty bedding and replaced, move his food bowl to where he usually eliminates (he will pick a new spot to eliminate). Next cage cleaning, move his hut to a different corner or section of the cage. Etc, for each cage cleaning. Moving things around makes for a more stimulating life anyways, and I know it sounds strange, but changing it up every once in a while keeps him from establishing " this is where MY food bowl is", "this is where I mark MY territory", "this is MY hut I hide in" etc...-- possessiveness basically. It's still his and smells like him but change does help; as a change happens in the brain when he has to re-establish where his things are and what his daily movement is. Rodents tend to establish habits that lead to compulsive behavior, and you want to eliminate that.
You can also get him a small animal play pen, for when you take him out of the cage. Having a second space he can explore, and establish as additional space that is "his" outside of his cage, may help him feel more secure. When putting him in the playpen, offer a hut for hiding and comfort - especially until he gets used to the space. The pen I'm thinking of is made by Kaytee, and comes with a mat to put under it, that's washable, so he doesn't have direct contact with your floor (and whatever else might be on it).
Good luck with Ace!
