Haven’t ran into this before. Is this acceptable?
There are a number of reasons to cover it with a more durable product.
Make you recommendations and move on.
I hope there is a roofing membrane on top of the deck. Why is the bottom insulated anyway?
Michel males a good point.
Observation: Exposed Dow Corning closed cell blue 1,1/2" inch rigid extruded insulation board.
Suspect Damage by UV rays. It is difficult to say how extensive the damage is. It can effect the R value and composition of the board.
There is also what appears to be a pathway for weather to be introduced and that is at the joints. Once moisture is underneath it is trapped by the rigid form.
NOTE: From manufactures: Rigid foam placed on the exterior side of foundation & walls must be protected against physical damage and UV degradation.
Recommend: Covering the material with a desired siding, wall board, stucco finish or sheathing.