Super job..told you its a PITA...
them slabs are marshalls saxon paving btw. Natural and buff
if you pressure wash them " think you already have" before laying they will look really nice as there made with crushed stone rather than dyed..so will look like new..you can use a chemical that kills algae ..50/50 bleach etc you should get everything off them.
if you lay them on a mortar bed semi wet you will get a better job than sharp sand and cement if you dont mind me saying..pond paving needs to be on a solid bonded bed for best result or the bed can wash out..
a good sharp sand cement mix will work ok tho..if thats your prefered method