Fixes invisible slimes.

As in title. From commit logs, cerulean, sepia, bluespace and pyrite slimes look like a /tg/ feature [1](https://github.com/tgstation/-tg-station/commit/0a5b23c078c4c8253caee265da3af25d23b8f2d5) [2](https://github.com/tgstation/-tg-station/commit/c66a06d177a45de3790faa28613d52c9d42289d1) that never came to Bay.