Can I sue my ex for not following health insurance guidelines of the divorce decree?

This needs to go back to court. You can ask the court to either require her to abide by the rules of your insurance plan and if she does not and takes the child outside the plan that she will be responsible for the additional cost to you. You should ask the court to have her repay you the $900 just to make the point, but it is unlikely that the court will do so this first time. Courts deal with this issue on a regular basis and most often when both parents have insurance policies the custodial parent's plan is primary and the noncustodial is secondary. You can ask the court to allow you to pay the difference in her single plan and a family plan and then excuse you from carrying the child on your plan. You are required to give her a copy of her insurance card but get this paperwork filed quickly.