In a dessert while night time,head to a group of mobs,2 creepers and 1 skeleton,kill both creepers,skeleton turns invisible and occasionally changes model to slime,shoot it with flame bow where it shakes then disapears, I have footage

I expected to skeleton to remain normal so I can kill it

Skeleton becomes invisible and resembles slime when jumping

Steps to Reproduce:
1. get shot by skeleton while killing creeper??
2. skeleton becomes invisible
3. changes into slime