You must determine whether or not you have standing to pursue any claims against the executor. Inheritance law varies from state to state, but as a general rule, the right to file a complaint against the executor is limited to those who have an interest in the probate proceeding. If the estate is being administered by an executor, this means the deceased left a will; therefore, actions against the executor will be limited to spouses and named beneficiaries in the will. Even if you aren't a named beneficiary, you might have standing to sue if you could benefit from the will being set aside.
As a preliminary step, you can draft and send a letter clearly identifying your complaint and explaining what it is you want the executor to do in order to rectify the situation. The job of executor is complex and often requires liquidating assets to satisfy claims against the estate; it can appear from the outside that the executor is lining his pockets with the deceased's property. The response you receive to your demand letter may clarify misunderstandings which, although easy to explain, could cost both you and the estate thousands of dollars in fees if the matter were pursued legally.
Compel an Accounting
If you are not satisfied with the executor's response, you can file a motion with the probate court to compel the executor to render an accouting of estate property, debts and assets. If the court grants your motion, the executor will have to come forth and show what he's been doing with the estate property. If there has been any improprieties, stealing or mismanagement, a forensic accountant may be able to help you identify this and determine whether further action is necessary. Even if the executor ignored your demand letter, the accounting might reveal no evidence of misbehavior.
Motion for Removal
You can file a motion to have the executor removed from his position, if necessary. Understand that as the moving party, the burden of proof will be on you to show that the executor has engaged in misconduct to such an extent that the court needs to remove him and appoint a new one. If the misconduct was bad enough, you might consider filing a separate lawsuit for damages against the executor in an attempt to recover lost value for the estate.