Given ccprog's translation, this looks to be the sort of signed photo one might commonly receive as a response to fan mail.
The issue here is that there's only a small window of his life during which that might have occurred. He wasn't particularly famous (or even over 20) before he joined the German military around the start of the war, so it would have to be "fan mail" owing to his Red Baron persona.
During the war and after he became famous, he was in a rather high command position where he had leadership, tactical, and training responsibilties that should have kept him far too busy to be dealing with fan mail.
There is one such window though. He suffered a head injury in July 6th of 1917. From then until the 23rd of October (with a brief attempt to return to duty for a bit more than a month around August), he was convalescing. During that latter period he wrote an autobiography. It would make sense if he were also answering fan mail during that period.
After that he went back into combat, and 6 months later was shot down over France.
However, for the entire duration of that window, the USA was at war with Germany. German immigrants were widely viewed as potentially disloyal, and many were being interned or even lynched. It seems highly unlikely anyone in the US would be so bold as to send the Red Barron fan mail during that time, and equally unlikely it would be received and faithfully delivered back.
I'd imagine that if one were to investigate the club's history, it was not operating as a German-language Singing club at that time.
This being said, the German Luftwaffe was promoting him as a heroic figure, along with his "flying circus", and it was said that he was recieving rather a lot of fan mail at squadron HQ. So it would make sense if there was staff assigned specifically to answer his fan mail. So on the theory this may have been handled by his staff and not the man himself, there would be a window from about early 1914 to the US entry in 1917 during which this might have been received and sent out. If that happened, I hope they hid it well from 1917-1919.