Can a judge from another state rule on child custody matters?

I live in Maryland with my 3 girls and my ex who lives in alaska, has motioned the court for full custody , can the alaskan judge rule, and what power does he have here in Maryland? Please help any feedback would be much appreciated.

There is a law called the "Interstate Child Custody Jurisdiction Act" which has been enacted by all US states. The essence of that law is that the state with which the children have the most contacts, and/or where the most evidence regarding the best interests of the children is located should assume jurisdiction - any other state should defer to that state.
There are exceptions - such as when children are moved from their 'home state' under an emergency to protect them, but the exceptions only allow for temporary custody orders until jurisdiction can be ruled on fully.
If you and the kids lived in Alaska for years before recently moving to Maryland, then Alaska courts may be able to assert jurisdiction. If you all lived in Maryland and your ex moved to Alaska, he has virtually no chance.
If there is any doubt about which state is the 'home' of the children, the UCCFJ requires that the judge where the custody motion has been filed to contact the appropriate court in the other state and try to work out which state is more appropriate under the standards of the Act.
I hope that is helpful to you. Good luck.

Actually its called the Uniform Child Custody Jurisdiction and Enforcement Act, however, it matter if the girls ever lived in Alaska and for how long, also a lot of other factors.

Generally only the court from the child's home state, or the state in which they live with their legal custodian will have jurisdiction, and that courts ruling will be recognized all over.
There are only 4 states where it is not enacted, and none of those are relevant to you.

If the girls are living with you and are with you now, he would have to file where you live in Maryland. If they are with him, get there now cause the judge can grant him custody.

This can not be done in Alaska, unless your original custody arangement was filed in Alaska.

you would have to take this case from Maryland..to Alaska since the children now live there and that judge rules the case