https://www.savingforcollege.com/article/yes-your-529-plan-will-affect-financial-aid
Account Ownership
The value of a 529 plan owned by a dependent student or one of their parents (529 plans do not allow joint ownership) is considered a parent asset on the FAFSA. About the first $10,000 will fall under the Asset Protection Allowance (the exact amount depends on the older parent’s age). Any parental assets beyond that amount will reduce a student’s aid package by up to a maximum of 5.64% of the asset’s value.
So, if a parent’s 529 account exceeds the Asset Protection Allowance by $10,000, his child’s financial aid award could be reduced by as much as $564. Of course, no one wants to lose $564, but the tax-free investment gains you’ve earned in your 529 account could likely outweigh this tiny loss. However, other student-owned assets are not treated as favorably. A custodial account under UGMA/UTMA, for example, will be counted as a student asset and will reduce the financial aid package by 20% of the asset value. So, in this case a $10,000 student asset means $2,000 less financial aid.
As mentioned above, assets in a 529 plan owned by a grandparentor other relative are not included on the FAFSA.