I have a DM on mobile with a failed slack message in it that has clogged my UI for months; literally months. On mobile it means that I can't tell if this user

I have a DM on mobile with a failed slack message in it that has clogged my UI for months; literally months. On mobile it means that I can't tell if this user has responded to a message or is online at all. I tried telling it not to send ages ago when it actually failed but it hasn't worked, and frankly now it's so far back in history it's impossible for me to try again. I've deleted slack and re-installed to no avail. It's really an awful problem.