What the biohazard symbol needs are human bodies implace on the spikey bit with their heads alternately melting and exploding. Now that’s a universal symbol.
Peace.
“OK, now draw a spike plunging into it’s eye.”
Some of the symbols on heavy equipment are pretty universal in their little ‘sausage-man’ drawings (like on public washroom doors)… a garbage truck has a picture of someone crouched in an enclosed space with his arms lifted, trying to keep the lid from crushing him; a wood-splitter has a drawing of the auger part with a guy twisted around it, his head and arms flailing at one end and his feet at the other.
And I don’t think a ‘recognisable’ symbol necessarily means a universal one. The Coke swirl and the Nike swoosh might be recognised by a large chunk of the world’s population, but it’s still cultural… if you haven’t been exposed to that aspect of culture you won’t know what it means.
A crescent moon is a pretty good one… I don’t think there’s anyone on the planet who will never see the moon throughout their whole life.
Given the universality of facial expressions… maybe the ‘have a nice day’ happy face would be universally recognised as a symbol of either happiness or friendship or welcome. 
It’s funny that we use the word ‘universal’, though… if we met people from another planet we’d have to find another word for ‘things automatically recognised by all Earth people’. Finding a symbol that was truly universal would be even more work. (Although if their planet had a round moon, they’d recognise the crescent moon shape too.)