I take it you mean flag stones? You can buy something to clean it with (Amazon sell all sorts) but it will need doing every couple of years of so. As for the buddleja they will grow anywhere and on anything and are the toughest of shrubs so I think it will be ok when the weather improves.

If your Buddleias are 'davidii', they need cutting down now anyway - take them down to within 2 inches of last year's wood. This should mean you're removing around 6-8 feet of topgrowth from a buddleia more than 2 years old, unless its a dwarf variety. They will put on that amount of growth in a season, so they'll need doing againj next year. The loss of leaves is normal - they are really deciduous plants, but keep some or all of their leaves in milder winters, and despite the snow and cold spells, this winter hasn't been that cold.

The cold, dry East wind has frazzled the leaves. Buddleja davidii is semi-deciduous and would normally keep some small leaves, depending on cultivar, but for this late cold spell. Pruning needs to be left until the coldest weather has passed, but it seems now that the worst has passed.

They should start to show developing buds any time now and these will grow rapidly. Other Buddleja species such as alternifolia and globosa (both late spring flowering) should be left well alone until after flowering.

I dont know of a permanent solution to keep your flag stones free from algae but a great product to restore them to their clean status is Algon. Available from most garden centres. As it is organic it keeps working and lasts for several seasons and is non toxic to animals and surrounding plants.

I have decking in my garden and have tried everything without any success.... I now put neat domestos all over it and brush it in with a soft sweeping brush, I leave it overnight and then rinse it off next day and it comes up beautiful. I have been doing this once a year for 5 years and No it has not rotted the decking, if you look at my very first blog you will see how nice it was last year. I just did it again last weekend and its came up beautiful again.

pressure hose......lots of fun and a good clean finish :)

Marion's solution works - but if you have paving or a deck where the run off of rinse water gets onto soil where there's planting, it will contaminate the soil. Algon, or a pressure wash, are safest. There is no permanent solution to the problem with flat laid stones or decking, such as in a patio - they will, particularly in shade during the winter, acquire a 'green coat'.