slab cleaner

Hi I’ve recently had a sandstone patio laid on compacted type 1 with wet bed by a landscaper. The last few days white marking is appearing on the slabs, if the slab is wet, these marks appear back really quickly, has anyone any idea what they are, what caused them and if they can be removed...

Hi all, anybody got any ideas how to clean up patio slab? had a go with my jet washer, need something to dissolve the moss etc. someone told me vinegar ? any suggestions would be gratefully received