The extra storage pool 255 is there by design, this is not a problem. The extra storage pool 255 is used to hold the DMCL.
It is allocated before startup reads the system definition from the dictionary. IDMS does not know how big the pool 255 should be so it allocates it a default size of 32MB. It's size can be controlled by SYSIDMS parameter CV_STARTUP_XA_REGION_MB. You only need to specify this parameter if your DMCL is very large. After startup any extra storage in this pool is released so it will always show as being 100% full.
This extra storage pool 255 is considered a dynamic extension of the storage pool 255 defined in the SYSGEN (its SCT control block will have the SCTDYNA flag set).