I blew up that pict and I don't think it's a sponge in my eyes it looks like your palys are starting to spread , I have been growing palys for ever , I started with 5 paly frag years ago now I have 1000's , god knows how many I gave away
they spread kind of fast I get about 8/10 heads a week I add small rocks so they grow onto them then sell them off , if you see them going in a direction you don't want you'll have to keep them cut back as they have a mind of there own , I just finally got mine to climb the back wall can't wait till they fill in I'm figuring around 3/6 months , I'm trying to get them going on the back wall on the other side of the tank but I've still haven't had luck of any to take yet
just ordered a few more frags of them in blue , green , red , tie dye can't wait to get them going
they just started down the rock on other side I'm going to head them off with some blue ones so they kind of blend
hope you have the same luck as me with them , I also started adding them into my predator tank just to add some extra life on the rocks