sponge filter turn green

[Crono]

is this filter alright to continue to be used?
I remember when I first buy it, it was bright yellow.
surf through the web, I read that could be algae in the filter, if not ammonia.

when I store it, its color was yellow. when I take it out(after a week store it), turns into this.
water and squeeze out, no change.

 

[esoxlucius]

Don't worry about it, it's just natural discolouration through use.

Even further down the line you'll find, with all the regular squeezing, that they may become slightly distorted too.

And even further down the line small particles of the sponge, leading to small clumps, will come away.

Time to replace them then.