The Slaadi incubation effect isn't a combat ability, it's how they reproduce
Combat challenge is determined by how much damage a monster can deal out over 3 rounds, and how hard they are for a party to take down (AC, HP etc). This ability does neither of these things and thus is not a combat challenge and doesn't impact CR.
Abilities like this, that don't factor into CR, pose an out of combat challenge to the party (as opposed to an in combat challenge).
The Death Slaad entry has some relevant lore (emphasis mine):
Death slaadi are suffused with energy from the Negative Energy Plane and exemplify evil’s corruption of chaos, and they take sadistic pleasure in bringing harm to others. They propagate their race by dragooning mobs of red and blue slaadi and invading other planes. Humanoids who survive the incursion become incubators for new slaadi.
The offspring of these incubations are red, blue or green Slaad.
Red/Blue Slaad are not very intelligent (with int scores of 6 & 7 respectively) and may indeed immediately attack sleeping party members as you describe.
Green Slaad are incubated if the interacted host could cast 3rd level spells or higher, they are also described as being
[...] surprisingly intelligent
As a result the party member who births/transforms into a Green Slaad wouldn't necessarily attack the rest of the party, but would likely leave and preserve itself.
It's also not clear that party members would know they have been infected.
Both red and blue Slaad have text which forces a constitution save:
If the target is a humanoid, it must succeed on a DC 15 Constitution saving throw or be infected with a disease called chaos phage.
If the target is a humanoid, it must succeed on a DC 14 Constitution saving throw or be infected with a disease—a minuscule slaad egg.
Constitution in game is described as measuring:
health, stamina, and vital force.
And specifically Constitution checks represent:
Constitution checks are uncommon, and no skills apply to Constitution checks, because the endurance this ability represents is largely passive rather than involving a specific effort on the part of a character or monster. A Constitution check can model your attempt to push beyond normal limits, however.
The DMG states describes saving throws as:
A saving throw is an instant response to a harmful effect and is almost never done by choice.
And that constitution saving throws should be used for:
Enduring a disease, poison, or other hazard that saps vitality
From these descriptions what the character will know is that the claw attack from a Red/Blue Slaad is in some way attacking their vitality (in a similar way to a poison would). They won't know that they've been infected with something until some other noticeable effect occurs to them (i.e. loss of hit points from the Blue or feeling unwell just prior to the birth from the Red).
What the player on the other hand knows is that they were told to make a Constitution Save and they were not poisoned and took no poison damage. This leaves them with two options:
- They made the save and avoided the threat
- They have been infected with something
Out of those two, #1 is by far the most common, and until they see other outward signs they will likely treat it as #1. Otherwise they are going to be wasting a lot of 3rd level spell slots for no particular reason (or gold if they don't have access to 3rd level spell slots directly).