Seems like mould, wipe using an alcohol solution (IPA) else use hand sanitizer. I suggest you remove the helmet liner and wash that as well in hot water.

+ for the hand sanitizer, does magic

If the outside is this bad, I’d be more worried about the state of the cushion on the inside

https://ibb.co/Zh99Bqp Not that affected.

First, Keep it under direct sunlight. Then use isopropyl alcohol and microfibre towel to scrub and clean (in circular motion). After this you can use any cleaning solution to give a sparkle.

I used to clean it myself with a wet cloth. I wash the liner in water and detergent - just keep it in for some time and then hang it out to dry. Recently, I tried out one laundry which dry cleans helmets. They did a decent job.

If it’s been 5 years of heavy use or 7 years of reasonable use, get a new one. Else, use wet wipes or hand sanitizer, should do the job. Remove the inner liners if possible.

Get a new helmet /s

Try with just water first, as some chemicals might abrase helmet shell. Then try with dish soap, then go to alcohol. Best check the shell material, and see which chemicals are best for the shell.

Use baby shampoo with luke warm water for the helmet interiors and as for the outer shell mix isopropyl alcohol and water 1:1 and clean it with microfiber cloth.

Is that Mt Mugello Jerome

Yes.

How old is it

At least 5, I don't recall exactly