Mayonnaise in stand mixer is very runny
I'm taking a stab here, but since you're accustomed to making mayonnaise in a food processor, you probably already have a habit for how you pour in the oil. Since a food processor moves so fast, it can emulsify quickly, so you can pour the oil in relatively quickly. 

A mixer is much slower, like using a whisk, you need to pour the oil very slowly, so every drop becomes completely emulsified before more is added. So try it slower next time.

For this mayonnaise, try to fix it as if it's broken, as in this video

Put another egg yolk in a mixing bowl and slowly add your thin mayo while whisking (or mixing in your mixer). Then slowly add more oil until the consistency is right.

BTW: See how slowly she's pouring?