As we know, Naruto (Jiraiya's apprentice) has mastered the Sage Mode of toads. Also, Kabuto (Orochimaru's apprentice) has mastered the Sage Mode of snakes. So, my question is, is there a Sage Mode of slugs (related to Tsunade)?
The Sage Mode of the slugs is somewhat related to the Sage Mode of Hashirama Senju (First Hokage). He learnt Sage Mode in Shikkotsu Woods/Forest, a legendary place that is equally famous as Mount Myōboku (toad) and Ryūchi Cave (snake). It is the home of the slug Katsuyu, parts of which are summoned by Tsunade and one of her disciples, Sakura Haruno.
In Hashirama's fight with Madara, it appears that he enters a Sage Mode (bottom right corner) similar to Tsunade's, with a diamond shape tatoo-like symbol in her forehead.
Though it is not explicitly said neither in the manga nor in the anime that Tsunade enters Sage Mode, it is clear that Hashirama did.
Sages use nature energy. Tsunades "100 healings technique" uses her own stored chakra.
Also, Hashirama does not use the same healing technique. His is called "Mitotic Regeneration" in his sage mode and it works by increasing the speed at which his cells replicate through mitosis which replaces the damaged or dead cells.
I'm too lazy to find the exact episode, I just watch/read a lot of Naruto..
I also assume that Hashirama used the Slugs-based Sage Mode. I noticed the name Shikkotsu Forest/Wood, "wood" is in the name of the jutsu Hashirama used - the Wood style. This observation makes me conclude that Hashirama is a Slug Sage. I also noticed that Hashirama got a healing jutsu which the slugs used. The jutsu was also inherited by his granddaughter (Tsunade-sama), who also passed the power to her apprentice (Sakura-chan).