For other characters, the same input sequence would be called a "reverse usmash".
For Squirtle, we add "hydroplane" into that term to describe the grounded boost of momentum he achieves. So it is called "reverse hydroplane usmash".
The term "hydroplane" itself has existed since Brawl, and while a bit silly sounding, it does make sense and combines well with common tech names to limit the amount of unique terms we need to come up with.
Technically it is cancelling a sling jump into an usmash, but the forward hydroplane usmash is also a jump cancelled usmash and we still call it hydroplane. We also do this with hydroplane fsmash and dsmash, which require pivots. So we determined that hydroplane describes cancelling Squirtle's turnrun into grounded momentum.
One of the trickiest name-related things for me are with hydroplane fsmashes. Since they require a pivot, they can only be hydroplane'd forward (during the rebound), but you are able to move in that direction while fsmashing in either direction. So it's a bit inconsistent to call it a "reverse hydroplane fsmash" when you fsmash in the direction opposite your momentum, since you'll always be moving forward, but I guess I use it and just hope the players I'm talking to understand that with fsmash you can only hydroplane in one direction out of turnrun, but you can start your attack in either direction.