shower drain cover corroded

i am having trouble removing a somewhat corroded shower drain cover from a fiberglass shower. i have been to lowe's and what they seem to sell are the snap-in variety of shower drain cover which this one is not, i believe, because there are no notches with which to remove the cover. looks like it is siliconed or caulked to the shower drain. i have tried prying underneath with a screwdriver and i accomplished nothing. i am looking for ideas to remove the darn thing, because i am stumped.

thanks,

R