Hey, Just a little update, The crack seems shorter but a little wider open, I'm not sure if this is a good thing or a bad thing
As others have mentioned. Check for thrush (you can't mistake the smell). It will look very dark, black even. Soft, even gooey. Smells really bad. If the frog is loose enough that you can slide a pick under it you might need to cut some away.
Personally I wouldn't use bleach. Vinegar works great. Thrush doesn't like the acid. Apple cider vinegar if you want a better flavor
I've had horse I bought arrive with thrush. One looked a bit like what your picture showed. Not as long a openning. Shorter, but wider towards the back. Nothing to cut away at the time, but I was able to dig out some, rinse the vinegar and then packed in cotton (gauze works too) soaked with tea tree oil. Cleared it up, but she had a couple of other spots that showed up as pockets on other feet that I needed to dig out and cut away the overlaying bit of frog. Over all it took me a month to get all her feet "right".
Establlished thrush will cause the frog to be sore, so be careful when getting any out, since it could hurt some. Fortunately horses general deal with pain better than we do.
It's nasty stuff and loves getting up inside the frog. Doesn't need oxygen, thrives in a moist invironment, but will do well in dry too once it's in.
I practice prevention by having a spray bottle of water and vinegar mix that I spray thier frogs with when I pickout their feet. I can't stand thrush.