So had my boat in the water on a seasonal mooring about 10 days since she was last out taken out for a hull clean (she's not antifouled)and I find lots of pink patches all underneath. There was nothing too sticky/hard growing on it, and all the usual stuff cleaned off easily but not the pink stains!!
If it was a "drying mooring" it will just be the staining from whatever she sat on at low water. A good hull cleaner should get it off, but Y10 would be my first port of call!
Hullabaloo
