However, back during the Vietnam War, 'MOONBEAM' was a static callword used by the battlestaff of an EC-130 Airborne Battlefield Command Control Center orbiting over a particular AOR. The EC-130 ABCCCs have been phased out, with the mission being picked-up by E-8 JSTARS aircraft.
While I don't think anyone has ever confirmed any E-8s using MOONBEAM, I do know that there has been some interest within the 116th Air Control Wing to learn the static callwords & callsigns of units whose mission/lineage is now part of the 116th ACW, for potential re-use as a sign of respect to their lineage. If/when they do this, other callwords they'd be using would include ALLEY CAT, BOOKSHELF, CRICKET, & HILLSBORO, with a front-end callsign of AXE (a play on ACCS -- Airborne Command & Control Squadron).
A prior respondent said a Google search revealed it was a possible EC-135. There are very few EC-135s still operating today, even if the original source was accurate (which I doubt).