why does jim tell huck that huck doesn't "walk like a girl"

Huck is disguising himself to look like a girl, so he can go across the river to the mainland and find out what's going on. Jim tells Huck he isn't walking like a girl because he isn't.... he simply looks like a boy in girl's clothing. He's correcting Huck's walk so he won't get caught.

Jim said nobody would know me, even in the daytime, hardly. I practiced around all day to get the hang of the things, and by and by I could do pretty well in them, only Jim said I didn’t walk like a girl; and he said I must quit pulling up my gown to get at my britches-pocket.