Fungus on rosemary

Does anyone have an idea what this is? I wished I’d taken a picture yesterday as it looked completely different - a cream colour tiny spores spread all over the small raised bed the rosemary is in. Then overnight it’s changed the this. Up until 2 weeks ago the rosemary was fantastic and growing all over the bed and decking but then half of it died and now this fungus thing is there. I’m going to dig it out but am just intrigued as to what it is and why it’s suddenly appeared.