Wood in terrariums is a bit of a nightmare to be honest! A little mould isn't necessarily a problem but if it's rapid growing I would remove it!

I find the best wood to use is cork bark but even this can get a little mouldy if kept too wet for too long.

It also sounds like you may be misting a little too much. Twice a day is the recommended unless you are trying to breed them!...the substrate should have chance to dry out a little between sprays and should never be sodden!