Is the credit a Credit Note or a Receive Money ?
Go to the Customers tab, click on the blue balance for the customer with the credit and then click edit next to the credit. Review the credit, has it been allocated to the correct Customer. Hopefully there is some information in the description fields.
If its a Credit Note which can’t be reallocated to another Customer, therefore somehow created in error, you could just delete it.
If its a Receive Money, go back to the bank statement around that date and validate that its an actual deposit and for which customer. If you discover that there is no related/valid banking then you could assume it was created in error and you could delete it.