My Terracotta Pots

This is probably just algae. the terracotta is porous so this will always happen on the shaded side. I think it adds character, but then maybe Im just lazy. Some people actually paint yoghurt on to the pots to get it to grow quicker.

They are very green and my bonnie new terracota pots in my medatarion area were looking so good and now too old and dirty looking.

The green is algae and will grow on the pots if you water the plants - and, yes, you are obviously going to water the plants. I don't know that you can prevent it, only clean it off. I use a scrubbing brush and bowl of water and clean carefully so as not to spoil the plant.
Pots should always be scrubbed when they are empty and allowed to dry. As well as green algae you may also find heavy deposits of a white crust round the top of the pot. This is limescale which can also be scrubed off but, when the pot is empty, I tie a cord through the pot and drop it into the rainbutt for a couple of days. The acid rain softens the lime and it comes off very easily.

I think it look's nice..but each to their own..lol

Yes I do clean when empty Bulbaholic but these ones have alpine palnts and grasses in them.
TW Dont think you woudl like to see these :)