I had a bta before but they never got close to it.They host anything, mine have hosted an open brain, xenia, frogspawn, and now duncan's. Nothing to worry about.
Mines are snowflake.Before I got a nem my clowns just decided the side of the overflow was their private spot. Oh and fyi the mushroom is the host. The fish is the one who is adding themselves uninvited LOL.
My current clown was captive bred and lived for about a year in the sump. When I moved her into the main display she swam like an anthia in the middle of the water column. Then 48 hrs later she identified the BTA and now will not go more than 5 inches away from it. btas normally host maroon clowns. So it's very possible for non-maroon clowns to ignore BTAs for a while before figuring it out.