Current HP, not maximum HP.
It could have a special interaction in Arena and FGs for only doing up to 50 damage. This clause could also be placed in the card text, if the community deems it necessary, for use outside of Arena.
I agree that chaining should be prevented. I'm not sure about having damage scale to cards drawn, as that's a bit similar to Nightmare. Instead, I offer this alternative: instead of "When this turn ends...", the clause "... Immediately end the turn." will be added.
(upped) You have 100 HP, play this twice. 55 HP --> 30 HP. You just generated 70 mark quanta. If your mark was earth, and you can play some stoneskins now, well well. With one, you are back at 100 HP. with a second, you will have about 170 HP, at least. Above all that you drawed a ton of cards, and would be able to play any stone dragon or shrieker in your hand this instant.
Let's see if preventing chaining fixes this problem.
Assume an enemy can get a 5 ATK creature out every turn, including the first. Assume the earliest you can use this card is during your second turn.
Turn 1: Enemy deals +5 damage. (You: 95 HP)
Turn 1: You do nothing.
Turn 2: Enemy deals +10 damage. (You: 85 HP)
Turn 2: You play 1 Senesce. +38
. (You: 47 HP)
Turn 3: Enemy deals +15 damage. (You: 32 HP)
Turn 3: If you play a Senesce, you'll die next turn.
I think the chaining fix solves the problem presented.