I can scarcely believe my own negligence. I must apologize for it. I somehow missed these two passages, one on the monkey and the other on the turtle, the second one right after the other. And the salt or lye or caustic soda (lan-tsha) here poses a danger to the turtle itself, and not, we must note, to the monkey paw it wants to eat.
The source is in the Zhijé Collection, vol. 2, the text entitled “Expressions of Speech Taught Symbolically.” This forms part of a trilogy called “Mahåmudrå Teachings: Three Cycles of Responses Employing Symbolic Actions of Body, Speech and Mind.”
Observe, as you will, that the monkey is once again connected with the unrelaxed mind. And the turtle? Well, you’ll see.
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As a symbolic way of saying that if you haven’t turned your mind away from sangsara, there is no need to stay in a retreat [p.156]
— “If you can’t relax the monkey’s mind there is no way to make his hands and feet stay still.”
As a symbolic way of saying that if you can’t get rid of desire, there is no need for doing the practices
— “Since turtles and moisture go together, what’s the use? It doesn’t recognize that the swirling water has lan-tsha in it. It has no more serious enemy than that.”
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Now possible interpretations seem to stack up on top of possible interpretations. If you’re going to ask me what it all means, I’ll just say, quoting something Padampa says in the exact same text, for my own purpose,
“I haven’t the slightest idea. Ask the turtle’s head.”
[B1] Phyag-rgya-chen-po Brda'i Skor Gsum, or, Brda'i Zhus-lan Skor Gsum. Vol. 2, pp. 138-178.
[B1a]Sku'i Rnam-dag Brdar Bstan-pa. Vol. 2, pp. 138-152.
[B1b]Gsung-gi Brjod-bya Brdar Bstan-pa. Vol. 2, pp. 153-164 (the pages are entirely out of order in the reprint edition, but they were put back in order on the basis of the microfilm of Trulzhik Rinpoche's manuscript).
[B1c]Thugs-kyi Dgongs-nyams Brdar Bstan-pa. Vol. 2, pp. 165-177.
I work from my own draft translation of these three texts, which form the first set of Responsa (zhu-lan) texts in the Zhijé Collection. It is very difficult to compare this to the content of Molk's translation (on his pp. 177-192), which appears to have been rearranged at will by the translator. (Of course it is possible, too, that he worked from an unpublished manuscript unknown to me.) B1c is at least partly included on his p. 188 ff., but its title seems to be missing.
The Tibetan passage starts on line 7 of p. 159, and continues on line 1 of p. 156 (trust me, it's true). I give the text in 'texto' style, with no orthographic emendations:
'khor ba las blo ma log na ri khrod du bsdad pa la dgos pa myed pa'i brda' ru /  spre'u 'i sems ma dal na rkang lag la bsdad dbang myi mchi' gsung //
'dod pa ma thongs na nyams su blangs pa la dgos pa myed pa'i brda' ru / ru rbal rlan dang 'grogs pas ci la phan lan tsha'i chu rgod ngo myi shes / dgra' ru de las gnad pa myed gsung //
Lion of Siddhas: The Life and Teachings of Padampa Sangye, translated by David Molk with Lama Tsering Wangdu Rinpoche, Snow Lion (Ithaca 2008).
 Molk, p. 186: “He said, ‘With the monkey mind not relaxed, the limbs have no capacity to remain still.’ To indicate that, if one does not turn the mind from samsara, there is no need for staying in retreat.” I searched out this translation (I couldn't find the one about the turtle & the lan-tsha) just so you will have another translation to compare. I made my own translation without being under any influence from this one.
 Molk, p. 185: “When asked the nature of the perfect ultimate mode of existence, he said, ‘I have no idea! Ask the head of the frogs.’ To indicate that transcendent wisdom of the mind is beyond expression.”