Eu consegui instalar o aplicativo ignorando as dependências e isso funciona. Mas eu tinha instalado o libXScrnSaver-devel.
zypper addrepo https://download.opensuse.org/update/leap/42.3/oss//openSUSE:Leap:42.3:Update.repo
zypper ref
zypper in libXScrnSaver-devel
zypper in slack-3.0.5-0.1.fc21.x86_64.rpm*
Loading repository data...
Reading installed packages...
Resolving package dependencies...
Problem: nothing provides libXScrnSaver needed by slack-3.0.5-0.1.fc21.x86_64
Solution 1: do not install slack-3.0.5-0.1.fc21.x86_64
Solution 2: break slack-3.0.5-0.1.fc21.x86_64 by ignoring some of its dependencies
Choose from above solutions by number or cancel [1/2/c] (c): 2