I recently bought some clay pots and put a one year old date palm in it using a potting mix of 1 year old compost, leaf mold and very few of bought horse-manure-mix. For drainage I used some stones.
Today I noticed that there is some white foam visible pn the outside of the pot. It looks like a fungi. I read somewhere that it could also be salpeter.
So what it is and is it dangerous for my plants?
EDIT: as requested I upload a photo: (click for a larger version)