The putty turns black and ugly around the exposed edges. So does the usual silicone get ugly after a while, and it can't be cleaned back to white with anything.
I agree with waiting till the putty settles, then scrape it back and use that stuff mentioned above that supposedly (many claims for that--not always correct "depending") it does not turn black after a while.
We get a lot of mould here over the wet winter, but other places it might not be so bad. Bleach is the usual cure, but it has no effect on silicone where the mould is underneath it.
