I would watch your air cells during candling. The state of them will give you the most important feedback. If the eggs appear to be drying down normally then the humidity level will be fine. Personally, 51% sounds a bit high but your hygrometer may be reading a bit off too. That is why watching the air space is best.
It's not harmfull as long as they don't stay there for the whole time. If the humidity is too high the eggs will not lose enough moisture during development. This can lead to chicks too large to hatch properly or ones that drown at hatch time due to too much liquid. A few days at too high of humidity will do no harm.
Good I just mopped up alot more water and the humidty dropped to 46%. They have probably been around in the 50's- lower 60's a couple of days. They have only been in the bator for 4 days.
Think of getting your humidity the first 18 days into a range of acceptable numbers. Anywhere between 35 and 50% is my own guide, trying never to go over 50%. My usual range is about 40-48%. Then on Day 18, I try to get it between 68-72%. I'm having good hatches this way right now.