In this verse Jesus is talking about the other side of the scroll “ Revelation “ The mysteries in and of Christ Jesus that “ He reveals “ hidden manna means hidden word . . .
Remember that Jesus is the Word; He is referring to himself - the manna (the body and blood of Jesus) is hidden from those who are not full Christians. When the Eucharist is mocked and scorned and 1-1/2 millennia of uncorrupted Christian understanding of the Real Presence was rejected by many, this is the hidden manna. Hidden not from the believers, but from the self-believers and those whose god is that which they have fashioned for themselves from the materials found in the earth.