How to get rid of these white marks on besser block?
Those white marks are mineral deposits from water that has leaked out through the wall. I suspect they'll be difficult to clean off a block wall.
Have you dried a scrubbing brush and a mix of warm water and dishwasher detergent? Failing that you might like to try Chemtech 1L Brick Tile And Paver Cleaner. You'd definitely want to do a test section in an inconspicuous location to make sure that the cleaning product doesn't discolour the brickwork.
Please let me know if you have any questions.