Wait, how would the croc not notice a furry centipede crawling up its nose?
I expect it will very much notice THAT part. We only need stealth to get in position to get into the nose or other oriface, and immediately start our attack. This is the essence of being a successful ambush predator - and essentially the same method that gained us our first mouse.
Theory is that this grime covered log is familiar with and unconcerned by algae/normal debris. If we move slowly enough as we get into position, then move quickly enough once in position, then we will hopefully appear to be a normal, harmless, and utterly unimportant part of the log's normal environment until we are in perfect position to quickly become overwhelmingly destructive and unremovable.
The nose may not be the best place to try. Most crocodilians have armored plates that seal their nostrils when they submerge. If the plate is open, good times. If the plate's closed, we may have a wait, in a spot where we could just by being there be annoying or detectable to the croc. Other great spots include the mouth and the anus. and if we encounter any recent wounds or poorly healed scars as we move over our ride those might be a high-probability-of-success location to attack too - though these critters can roll and bash with the very best of them... so I'd rather we started from inside and next to an absolutely vital bit of the critter, where it's amazingly hard to bash or scrape something away from.
Imagine burning terrible agony from your hand or foot, as if you'd dipped the part in fire. You'd fling the limb savagely around, maybe purposefully bash it against the things around you in an attempt to knock off the pain, and possibly even try to destroy or sever the limb to escape the pain. Now imagine this pain starting inside your mouth, nose, or other orifaces. You'd still whip your body/head around, you'd flail... but you're far less likely to be able to bash your core or your head against obstacles as fast as you can - and if you do, you won't keep doing it for long. You can claw at yourself, but the parts you are clawing at are essential for your own life - you're helping to speed your death if you try to fight the pain that way, and you're very unlikely to successfully rip your head or body open and extract the problem attacking from within - as you maybe could if we were attacking an existing wound.
So, once we are in position, we wait for an opening if it's not already there, then move with savage speed, cling, and start digestion as fast as our little self can.
3 cm long is hopefully less than 3 cm wide or 3 cm high, we are SMALL. Small enough to have a grand chance to slide between this log's teeth, small enough with any luck to be able to fit (rapidly!) into a nostril. though I'd feel safest attacking the mouth. Very likely to be OPEN slightly, crocdilian mouths cannot completely seal even when closed.