Keeping Usha’s existence a secret would likely be close to impossible. You could shove her into your bag after feeding and prevent her from going out, but that’s... Kind of harsh, but maybe showing discipline early is a good idea? Gods, what am I thinking? I’m no mother!

She’ll start whining for food soon enough; for now, she was too occupied with making strange faces at Aryel. ''Guess she’s finding you interesting.''
''Ah! You’ve got a perceptive little lass.'' He laughed, then turned half-around to examine the outside world. ''I can’t see anybody looking for us if that has you worried.'' Huh, Usha really is fascinated by him for some reason.

Your blue friend wasn’t exactly glaring holes into the man. You’d say it might be... something like a crush? No, maybe she’s interested in him. The guy was in half-plate armor with a sword and a glaive; he did strike an unusual presence.

''We can spare some time for you to take care of your little one.'' Usha is not my daughter, Shii!
''I know, I know...'' Having a stranger here did make things a tad more complicated.

Two choices to make.
>You will walk around without hiding little Usha. She's a curious little lass who deserves to see the world, you're not going to be afraid of a little complication down the line.
>It'll be better for you to show some discipline. Usha should remain inside your bag when you're in the village. She won't like this, but you should start early by getting a little bit of control over her.
>Have her stay on Shigeko. It'll be a good way to obfuscate the truth and you might teach her that your blue friend is a source of safety, too.
+
>Have Shigeko take Aryel out of the house while you feed Usha. The slime's magical consuming nature is something you need to keep secret for as long as you can.
>Show some good faith and allow the man to witness Usha consuming the Aegis you'll summon to feed Usha. You've never been good at keeping secrets... it's better not to try.