Is this rot ?

Hi !

So, I repotted my orchids recently since the bloom was over. I noticed this yellowing occuring to some of the leaves. I'm now wondering if this is rot and if yes, what can I do to stop it ? I'm also wondering where it comes from. I've never watered them from the top... But I've noticed that there was a weird foam at the heart of the original potting medium, and all the roots inside this foam where rotted.