Some sort of fungus?

[Containergardener]

This has appeared in patches on one side of the gravel. How can I get rid of it? I've not had this before.

[veggiechicken]

Maybe one of the slime moulds - there are several including the delightfully named "Dog vomit slime mould".
They come, they go, nothing really to do about them.

[Containergardener]

Ok , thanks

[mcsee]

I'd try a splash of Bleach on it, nothing to lose. Just looks unsightly.

[Jungle Jane]

I’d do nothing to it,but if it was by the front door where people knock at your door I’d spray it with water maybe,I just found this science article about fungi attaching to rocks & extracting minerals & moving them through the soil ecosystems,sounds beneficial for the earth -
Iron-eating fungus disintegrates rocks with acid and cellular knives | Science | AAAS

[keat63]

I'd also say 'dog vomit'
Bleach or mold & mildew cleaner will sort it.