Absolutely fine. will not harm any fish.
Some fish will eat it, as will some snails.
If none do then your only problem is if you find it un-aesthetically pleasing to look at. The best way to get rid of it in this case would be to scrub it off and then boil the wood itself. This generally works perfectally.
It is usually described as a mould or as a fungus but it is actually because the wood still has some living properties and although it cannot grow it produces this mould-like slimey white mucus.
If it doesn't bother you, leave it. It'll most likley clear itself up!
Hope this helps!