I have had the no sealing problem and it was solved by adding sealant. I think the sealant was about gone though.
My understanding is the issue with eating rims has been addressed for the most part. I 'm a relative newbie with tubeless, but I have been using it for 4-5 years and have had no such problems
I would never clean it up unless it was stringy and interfering with sealing. Never seen that though, usually it dries on the tire